f2fs: introduce F2FS_I_SB, F2FS_M_SB, and F2FS_P_SB

This patch adds three inline functions to clean up dirty casting codes.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

diff --cc fs/f2fs/checkpoint.c
Simple merge

diff --cc fs/f2fs/data.c
Simple merge

diff --cc fs/f2fs/dir.c
Simple merge

diff --cc fs/f2fs/f2fs.h
Simple merge

diff --cc fs/f2fs/file.c
Simple merge

diff --cc fs/f2fs/inline.c
Simple merge

diff --cc fs/f2fs/inode.c
Simple merge

diff --cc fs/f2fs/namei.c
Simple merge

diff --cc fs/f2fs/node.c
Simple merge

diff --cc fs/f2fs/node.h
Simple merge

diff --cc fs/f2fs/recovery.c
Simple merge

diff --cc fs/f2fs/segment.c
Simple merge

diff --cc fs/f2fs/segment.h
Simple merge

diff --cc fs/f2fs/xattr.c
Simple merge