xfs: verify icount in superblock write

Add a helper predicate to check the inode count for sanity, then use it
in the superblock write verifier to inspect sb_icount.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Bill O'Donnell <billodo@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>