[XFS] Factor xfs_itobp() and xfs_inotobp().

The only difference between the functions is one passes an inode for the
lookup, the other passes an inode number. However, they don't do the same
validity checking or set all the same state on the buffer that is returned
yet they should.

Factor the functions into a common implementation.

SGI-PV: 970925
SGI-Modid: xfs-linux-melb:xfs-kern:30500a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>