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

In the Linux kernel, the following vulnerability has been resolved:

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>