nilfs2: convert nilfs_super_block to kernel-doc

nilfs2: convert nilfs_super_block to kernel-doc

Eliminate 40+ kernel-doc warnings in nilfs2_ondisk.h by converting
all of the struct member comments to kernel-doc comments.

Fix one misnamed struct member in nilfs_direct_node.

Object files before and after are the same size and content.

Examples of warnings:
Warning: include/uapi/linux/nilfs2_ondisk.h:202 struct member 's_rev_level'
 not described in 'nilfs_super_block'
Warning: include/uapi/linux/nilfs2_ondisk.h:202 struct member
 's_minor_rev_level' not described in 'nilfs_super_block'
Warning: include/uapi/linux/nilfs2_ondisk.h:202 struct member 's_magic'
 not described in 'nilfs_super_block'
Warning: include/uapi/linux/nilfs2_ondisk.h:202 struct member 's_bytes'
 not described in 'nilfs_super_block'
Warning: include/uapi/linux/nilfs2_ondisk.h:202 struct member 's_flags'
 not described in 'nilfs_super_block'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Viacheslav Dubeyko <slava@dubeyko.com>