xfs: remove pointless shadow variable from xfs_difree_inobt

In xfs_difree_inobt, the pag passed in was previously used to look up
the AGI buffer.  There's no need to extract it again, so remove the
shadow variable and shut up -Wshadow.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>