xfs: consolidate extent swap code

So we don't need xfs_dfrag.h in userspace anymore, move the extent
swap ioctl structure definition to xfs_fs.h where most of the other
ioctl structure definitions are.

Now that we don't need separate files for extent swapping, separate
the basic file descriptor checking code to xfs_ioctl.c, and the code
that does the extent swap operation to xfs_bmap_util.c.  This
cleanly separates the user interface code from the physical
mechanism used to do the extent swap.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>