f2fs: fix NULL pointer dereference in f2fs_submit_page_write()

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

f2fs: fix NULL pointer dereference in f2fs_submit_page_write()

BUG: kernel NULL pointer dereference, address: 0000000000000014

RIP: 0010:f2fs_submit_page_write+0x6cf/0x780 [f2fs]

Call Trace:

<TASK>

? show_regs+0x6e/0x80

? __die+0x29/0x70

? page_fault_oops+0x154/0x4a0

? prb_read_valid+0x20/0x30

? __irq_work_queue_local+0x39/0xd0

? irq_work_queue+0x36/0x70

? do_user_addr_fault+0x314/0x6c0

? exc_page_fault+0x7d/0x190

? asm_exc_page_fault+0x2b/0x30

? f2fs_submit_page_write+0x6cf/0x780 [f2fs]

? f2fs_submit_page_write+0x736/0x780 [f2fs]

do_write_page+0x50/0x170 [f2fs]

f2fs_outplace_write_data+0x61/0xb0 [f2fs]

f2fs_do_write_data_page+0x3f8/0x660 [f2fs]

f2fs_write_single_data_page+0x5bb/0x7a0 [f2fs]

f2fs_write_cache_pages+0x3da/0xbe0 [f2fs]

...

It is possible that other threads have added this fio to io->bio

and submitted the io->bio before entering f2fs_submit_page_write().

At this point io->bio = NULL.

If is_end_zone_blkaddr(sbi, fio->new_blkaddr) of this fio is true,

then an NULL pointer dereference error occurs at bio_get(io->bio).

The original code for determining zone end was after "out:",

which would have missed some fio who is zone end. I've moved

this code before "skip:" to make sure it's done for each fio.