xfs: refactor xfs_difree() inobt bits into xfs_difree_inobt() helper

Refactor xfs_difree() in preparation for the finobt. xfs_difree()
performs the validity checks against the ag and reads the agi
header. The work of physically updating the inode allocation btree
is pushed down into the new xfs_difree_inobt() helper.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>