xfs: remove unnecessary shifts

The superblock verifier already validates that (1 << blocklog) ==
blocksize, so use the value directly instead of doing math.

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