f2fs: f2fs_do_map_lock

Split f2fs_do_map_lock into a lock and unlock helper to make the code
using it easier to read.

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