xfs: fix unused log variable in xfs_log_cover()

The log variable is only used in kernels with asserts enabled.
Remove it and open code the dereference to avoid unused variable
warnings.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>