erofs: fix use-after-free on sbi->sync_decompress

z_erofs_decompress_kickoff() can race with filesystem unmount, causing
a use-after-free on sbi->sync_decompress.

When I/O completes, z_erofs_endio() calls z_erofs_decompress_kickoff()
to queue z_erofs_decompressqueue_work() asynchronously. Then, after all
folios are unlocked, unmount workflow can proceed and sbi will be freed
before accessing to sbi->sync_decompress.

Thread (unmount)        I/O completion        kworker
                        queue_work
                                              z_erofs_decompressqueue_work
                                               (all folios are unlocked)
cleanup_mnt
 ..
 erofs_kill_sb
  erofs_sb_free
   kfree(sbi)
                        access sbi->sync_decompress  // UAF!!

Fixes: 40452ffca3c1 ("erofs: add sysfs node to control sync decompression strategy")
Reported-by: syzbot+52bae5c495dbe261a0bc@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=52bae5c495dbe261a0bc
Reviewed-by: Chao Yu <chao@kernel.org>
Reviewed-by: Jianan Huang <jnhuang95@gmail.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>