xfs: fix unused variable warning in xfs_buf_set_ref()

Fix an unused variable warning on non-DEBUG builds introduced by
commit 7561d27e

 ("xfs: buffer lru reference count error injection
tag").

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>