f2fs: add superblock and major in-memory structure

This adds the following major in-memory structures in f2fs. - f2fs_sb_info: contains f2fs-specific information, two special inode pointers for node and meta address spaces, and orphan inode management. - f2fs_inode_info: contains vfs_inode and other fs-specific information. - f2fs_nm_info: contains node manager information such as NAT entry cache, free nid list, and NAT page management. - f2fs_node_info: represents a node as node id, inode number, block address, and its version. - f2fs_sm_info: contains segment manager information such as SIT entry cache, free segment map, current active logs, dirty segment management, and segment utilization. The specific structures are sit_info, free_segmap_info, dirty_seglist_info, curseg_info. In addition, add F2FS_SUPER_MAGIC in magic.h.

JKJaegeuk Kim committed 13 years ago