f2fs: data: fix warning Using plain integer as NULL pointer

changed passing function argument "0 to NULL" to fix below sparse
warning

fs/f2fs/data.c:426:47: warning: Using plain integer as NULL pointer

Signed-off-by: Hariprasad Kelam <hariprasad.kelam@gmail.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>