f2fs: fix kernel crash due to null io->bio

We should return when io->bio is null before doing anything. Otherwise, panic.

BUG: kernel NULL pointer dereference, address: 0000000000000010
RIP: 0010:__submit_merged_write_cond+0x164/0x240 [f2fs]
Call Trace:
 <TASK>
 f2fs_submit_merged_write+0x1d/0x30 [f2fs]
 commit_checkpoint+0x110/0x1e0 [f2fs]
 f2fs_write_checkpoint+0x9f7/0xf00 [f2fs]
 ? __pfx_issue_checkpoint_thread+0x10/0x10 [f2fs]
 __checkpoint_and_complete_reqs+0x84/0x190 [f2fs]
 ? preempt_count_add+0x82/0xc0
 ? __pfx_issue_checkpoint_thread+0x10/0x10 [f2fs]
 issue_checkpoint_thread+0x4c/0xf0 [f2fs]
 ? __pfx_autoremove_wake_function+0x10/0x10
 kthread+0xff/0x130
 ? __pfx_kthread+0x10/0x10
 ret_from_fork+0x2c/0x50
 </TASK>

Cc: stable@vger.kernel.org # v5.18+
Fixes: 64bf0ee ("f2fs: pass the bio operation to bio_alloc_bioset")
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/data.c

@@ -653,6 +653,9 @@ static void __f2fs_submit_merged_write(struct f2fs_sb_info *sbi,
 
 	f2fs_down_write(&io->io_rwsem);
 
+	if (!io->bio)
+		goto unlock_out;
+
 	/* change META to META_FLUSH in the checkpoint procedure */
 	if (type >= META_FLUSH) {
 		io->fio.type = META_FLUSH;
@@ -661,6 +664,7 @@ static void __f2fs_submit_merged_write(struct f2fs_sb_info *sbi,
 			io->bio->bi_opf |= REQ_PREFLUSH | REQ_FUA;
 	}
 	__submit_merged_bio(io);
+unlock_out:
 	f2fs_up_write(&io->io_rwsem);
 }