f2fs: add a f2fs_get_block_locked helper

This allows to keep the f2fs_do_map_lock based locking scheme
private to data.c.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>