stack trace changes

[Martin Hunt]

I've checked in a rewrite of the stack trace code.  I rewrote i386 and
x86_64 only. ppc64 and ia64 should still work as before, unless I made
any errors while reorganizing the code.

In 2.6.18 there is code to use DWARF to accurately unwind the stacks.
The new code uses this if available, otherwise it falls back to the
inaccurate unwinder.  Some situations like kretprobe trampolines confuse
the unwinder and it does what it can accurately then uses the inaccurate
unwinder. In all cases, it now says when the trace in not accurate.

I also added some related functions:
stack_size() - stack size
stack_used() - bytes of stack used
stack_unused() - bytes of stack unused
caller() - for kretprobes only right now, returns the calling function
name and address.
caller_addr() - ditto but returns an address only

Example output looks like this:

WITH NEW UNWINDER

(normal probe)
0xc04a28ac : meminfo_read_proc+0xf/0x313 []
 0xc04a01ad : proc_file_read+0x110/0x237 []
 0xc046f77c : vfs_read+0xa6/0x157 []
 0xc046fbeb : sys_read+0x41/0x67 []
 0xc0403f97 : syscall_call+0x7/0xb []

(normal probe when a return probe is on the same function)
0xc04a1bc3 : uptime_read_proc+0xe/0x99 []
 0xc06143c5 : kretprobe_trampoline+0x0/0x2b []
 0x00000c00
Leftover inexact backtrace:
 0xc04a009d : proc_file_read+0x0/0x237 []
 0xc046f77c : vfs_read+0xa6/0x157 []
 0xc046fbeb : sys_read+0x41/0x67 []
 0xc0403f97 : syscall_call+0x7/0xb []

(return probe)
Returning from: 0xc04a1bb5 : uptime_read_proc+0x0/0x99 []
Returning to: 0xc04a01ad : proc_file_read+0x110/0x237 []
Inexact backtrace:
 0xc04a009d : proc_file_read+0x0/0x237 []
 0xc046f77c : vfs_read+0xa6/0x157 []
 0xc046fbeb : sys_read+0x41/0x67 []
 0xc0403f97 : syscall_call+0x7/0xb []

WITHOUT NEW UNWINDER

0xc04a1c54 : meminfo_read_proc+0xf/0x27a []
Inexact backtrace:
 0xc048091e : dput+0x35/0x230 []
 0xc0479907 : __link_path_walk+0xbf4/0xd31 []
 0xc048091e : dput+0x35/0x230 []
 0xc0484fad : mntput_no_expire+0x11/0x6e []
 0xc0479af7 : link_path_walk+0xb3/0xbd []
 0xc06181fe : _read_unlock_irq+0x5/0x7 []
 0xc044c938 : find_get_page+0x3a/0x40 []
 0xc044f103 : filemap_nopage+0x192/0x31f []
 0xc0451564 : get_page_from_freelist+0x2a8/0x411 []
 0xc045bb40 : vma_adjust+0x3c8/0x43e []
 0xc045173a : __alloc_pages+0x6d/0x2c2 []
 0xc045c04f : vma_merge+0x18a/0x19a []
 0xc04a1c45 : meminfo_read_proc+0x0/0x27a []
 0xc04a028a : proc_file_read+0x110/0x237 []
 0xc04a017a : proc_file_read+0x0/0x237 []
 0xc046bac2 : vfs_read+0xa6/0x14e []
 0xc046bf26 : sys_read+0x41/0x67 []
 0xc0403e3f : syscall_call+0x7/0xb []

0xc04a1ecd : uptime_read_proc+0xe/0x99 []
Inexact backtrace:
 0xc06183fe : debug_stack_correct+0x1e/0x24 []
 0xc04a1ebf : uptime_read_proc+0x0/0x99 []
 0xc04a1ebf : uptime_read_proc+0x0/0x99 []
 0xc06188cf : kretprobe_trampoline+0x0/0x2b []
 0xc04a017a : proc_file_read+0x0/0x237 []
 0xc046bac2 : vfs_read+0xa6/0x14e []
 0xc046bf26 : sys_read+0x41/0x67 []
 0xc0403e3f : syscall_call+0x7/0xb []

Returning from: 0xc04a1ebf : uptime_read_proc+0x0/0x99 []
Returning to: 0xc04a028a : proc_file_read+0x110/0x237 []
Inexact backtrace:
 0xc04a017a : proc_file_read+0x0/0x237 []
 0xc046bac2 : vfs_read+0xa6/0x14e []
 0xc046bf26 : sys_read+0x41/0x67 []
 0xc0403e3f : syscall_call+0x7/0xb []