Get d_path's root natively with @cast

Get d_path's root natively with @cast

As mjw's aborted commit 5ab0b1b6 attempted, we can use a @cast to get
the filesystem root for the current task instead of embedded-C.  I've
left it using the kernel debuginfo instead of headers for now, because
all of the neighboring @casts are doing the same.

* tapset/dentry.stp (d_path): Use @cast to get the root.
  (__dentry_get_current_root): Remove, no longer needed.
* testsuite/buildok/dentry.stp: Test that dentry's functions all build.