f2fs: fix to handle error path correctly in f2fs_map_blocks

In f2fs_map_blocks(), we should bail out once __allocate_data_block()
failed.

Fixes: f847c699cff3 ("f2fs: allow out-place-update for direct IO in LFS mode")
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>