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: 64bf0eef ("f2fs: pass the bio operation to bio_alloc_bioset")
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>