[PATCH] Fix traceaio with IO_CMD_{PREAD,PWRITE}

[Frank Ch. Eigler]

Hi -

> This works:
> 
> global IO_CMD_PREADV = 7
> global IO_CMD_PWRITEV = 8
> 
> Do we have a better way to handle this?

Within a tapset snippet, code can use
   @const("IO_CMD_PREADV")
referring to an identifier in embedded-C scope.

If the IO_CMD_PREADV identifier were an enum value that shows up in
DWARF, then $IO_CMD_PREADV can resolve in recent versions of stap.

If none of the above, yeah, use the global gadget.

- FChE