f2fs: fix to avoid use-after-free issue in f2fs_filemap_fault

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

f2fs: fix to avoid use-after-free issue in f2fs_filemap_fault

syzbot reports a f2fs bug as below:

BUG: KASAN: slab-use-after-free in f2fs_filemap_fault+0xd1/0x2c0 fs/f2fs/file.c:49
Read of size 8 at addr ffff88807bb22680 by task syz-executor184/5058

CPU: 0 PID: 5058 Comm: syz-executor184 Not tainted 6.7.0-syzkaller-09928-g052d534373b7 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 11/17/2023
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0x1e7/0x2d0 lib/dump_stack.c:106
print_address_description mm/kasan/report.c:377 [inline]
print_report+0x163/0x540 mm/kasan/report.c:488
kasan_report+0x142/0x170 mm/kasan/report.c:601
f2fs_filemap_fault+0xd1/0x2c0 fs/f2fs/file.c:49
__do_fault+0x131/0x450 mm/memory.c:4376
do_shared_fault mm/memory.c:4798 [inline]
do_fault mm/memory.c:4872 [inline]
do_pte_missing mm/memory.c:3745 [inline]
handle_pte_fault mm/memory.c:5144 [inline]
__handle_mm_fault+0x23b7/0x72b0 mm/memory.c:5285
handle_mm_fault+0x27e/0x770 mm/memory.c:5450
do_user_addr_fault arch/x86/mm/fault.c:1364 [inline]
handle_page_fault arch/x86/mm/fault.c:1507 [inline]
exc_page_fault+0x456/0x870 arch/x86/mm/fault.c:1563
asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:570

The root cause is: in f2fs_filemap_fault(), vmf->vma may be not alive after
filemap_fault(), so it may cause use-after-free issue when accessing
vmf->vma->vm_flags in trace_f2fs_filemap_fault(). So it needs to keep vm_flags
in separated temporary variable for tracepoint use.