f2fs: update several comments

Update several comments:
1. use f2fs_{un}lock_op install of mutex_{un}lock_op.
2. update comment of get_data_block().
3. update description of node offset.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

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/node.c
Simple merge

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