erofs: fix UAF issue for file-backed mounts w/ directio option

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

erofs: fix UAF issue for file-backed mounts w/ directio option

[ 9.269940][ T3222] Call trace:

[ 9.269948][ T3222] ext4_file_read_iter+0xac/0x108

[ 9.269979][ T3222] vfs_iocb_iter_read+0xac/0x198

[ 9.269993][ T3222] erofs_fileio_rq_submit+0x12c/0x180

[ 9.270008][ T3222] erofs_fileio_submit_bio+0x14/0x24

[ 9.270030][ T3222] z_erofs_runqueue+0x834/0x8ac

[ 9.270054][ T3222] z_erofs_read_folio+0x120/0x220

[ 9.270083][ T3222] filemap_read_folio+0x60/0x120

[ 9.270102][ T3222] filemap_fault+0xcac/0x1060

[ 9.270119][ T3222] do_pte_missing+0x2d8/0x1554

[ 9.270131][ T3222] handle_mm_fault+0x5ec/0x70c

[ 9.270142][ T3222] do_page_fault+0x178/0x88c

[ 9.270167][ T3222] do_translation_fault+0x38/0x54

[ 9.270183][ T3222] do_mem_abort+0x54/0xac

[ 9.270208][ T3222] el0_da+0x44/0x7c

[ 9.270227][ T3222] el0t_64_sync_handler+0x5c/0xf4

[ 9.270253][ T3222] el0t_64_sync+0x1bc/0x1c0

EROFS may encounter above panic when enabling file-backed mount w/

directio mount option, the root cause is it may suffer UAF in below

race condition:

- z_erofs_read_folio wq s_dio_done_wq

- z_erofs_runqueue

- erofs_fileio_submit_bio

- erofs_fileio_rq_submit

- vfs_iocb_iter_read

- ext4_file_read_iter

- ext4_dio_read_iter

- iomap_dio_rw

: bio was submitted and return -EIOCBQUEUED

- dio_aio_complete_work

- dio_complete

- dio->iocb->ki_complete (erofs_fileio_ki_complete())

- kfree(rq)

: it frees iocb, iocb.ki_filp can be UAF in file_accessed().

- file_accessed

: access NULL file point

Introduce a reference count in struct erofs_fileio_rq, and initialize it

as two, both erofs_fileio_ki_complete() and erofs_fileio_rq_submit() will

decrease reference count, the last one decreasing the reference count

to zero will free rq.