[XFS] Don't assert if trying to mount with blocksize > pagesize

If we don't do the blocksize/PAGESIZE check before calling
xfs_sb_validate_fsb_count() we can assert if we try to mount with a
blocksize > pagesize. The assert is valid so leave it and just move the
blocksize/pagesize check earlier.

SGI-PV: 983734

SGI-Modid: xfs-linux-melb:xfs-kern:31365a

Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: David Chinner <dgc@sgi.com>