f2fs: Allow sbi to be NULL in f2fs_printk

At the parsing phase of the new mount api, sbi will not be
available. So here allows sbi to be NULL in f2fs log helpers
and use that in handle_mount_opt().

Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
[sandeen: forward port]
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>