f2fs: add f2fs_map_blocks

This patch introduces f2fs_map_blocks structure likewise ext4_map_blocks.
Now, f2fs uses f2fs_map_blocks when handling get_block.

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

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

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

diff --cc include/trace/events/f2fs.h
Simple merge