mkfs: inode/block size error messages confusing

mkfs: inode/block size error messages confusing

As reported by Brain:

# ./mkfs/mkfs.xfs -f /dev/test/scratch -b size=512
illegal inode size 512
allowable inode size with 512 byte blocks is 256
# ./mkfs/mkfs.xfs -f /dev/test/scratch -b size=512 -i size=256
Minimum inode size for CRCs is 512 bytes
Usage: mkfs.xfs
...

Fix mkfs to catch the block size as being too small, rather than
leaving it for inode size detection to enforce.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>