nilfs2: super root size should change depending on inode size

The size of super root structure depends on inode size, so
NILFS_SR_BYTES macro should be a function of the inode size.  This
fixes the issue.

Even though a different size value will be written for a possible
future filesystem with extended inode, but fortunately this does not
break disk format compatibility.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>