f2fs: fix to avoid out-of-bounds memory access

butt3rflyh4ck <butterflyhuangxx@gmail.com> reported a bug found by
syzkaller fuzzer with custom modifications in 5.12.0-rc3+ [1]:

 dump_stack+0xfa/0x151 lib/dump_stack.c:120
 print_address_description.constprop.0.cold+0x82/0x32c mm/kasan/report.c:232
 __kasan_report mm/kasan/report.c:399 [inline]
 kasan_report.cold+0x7c/0xd8 mm/kasan/report.c:416
 f2fs_test_bit fs/f2fs/f2fs.h:2572 [inline]
 current_nat_addr fs/f2fs/node.h:213 [inline]
 get_next_nat_page fs/f2fs/node.c:123 [inline]
 __flush_nat_entry_set fs/f2fs/node.c:2888 [inline]
 f2fs_flush_nat_entries+0x258e/0x2960 fs/f2fs/node.c:2991
 f2fs_write_checkpoint+0x1372/0x6a70 fs/f2fs/checkpoint.c:1640
 f2fs_issue_checkpoint+0x149/0x410 fs/f2fs/checkpoint.c:1807
 f2fs_sync_fs+0x20f/0x420 fs/f2fs/super.c:1454
 __sync_filesystem fs/sync.c:39 [inline]
 sync_filesystem fs/sync.c:67 [inline]
 sync_filesystem+0x1b5/0x260 fs/sync.c:48
 generic_shutdown_super+0x70/0x370 fs/super.c:448
 kill_block_super+0x97/0xf0 fs/super.c:1394

The root cause is, if nat entry in checkpoint journal area is corrupted,
e.g. nid of journalled nat entry exceeds max nid value, during checkpoint,
once it tries to flush nat journal to NAT area, get_next_nat_page() may
access out-of-bounds memory on nat_bitmap due to it uses wrong nid value
as bitmap offset.

[1] https://lore.kernel.org/lkml/CAFcO6XOMWdr8pObek6eN6-fs58KG9doRFadgJj-FnF-1x43s2g@mail.gmail.com/T/#u

Reported-and-tested-by: butt3rflyh4ck <butterflyhuangxx@gmail.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
