xfs_io: fix inode command help and argsmax

xfs_io: fix inode command help and argsmax

The short help implied that -n and -v were exclusive, and the longer
help wasn't particularly clear.

Further, argsmax is wrong; "-n -v num" is 3, not 2.

 # xfs_io -c "inode -n -v 123" /mnt/test2
 bad argument count 3 to inode, expected between 0 and 2 arguments
 # xfs_io -c "inode -vn 123" /mnt/test2
 128:32

Fix up all of those issues.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>