f2fs: fix to avoid out-of-bounds access in f2fs_truncate_inode_blocks()

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

f2fs: fix to avoid out-of-bounds access in f2fs_truncate_inode_blocks()

syzbot reports an UBSAN issue as below:

------------[ cut here ]------------
UBSAN: array-index-out-of-bounds in fs/f2fs/node.h:381:10
index 18446744073709550692 is out of range for type '__le32[5]' (aka 'unsigned int[5]')
CPU: 0 UID: 0 PID: 5318 Comm: syz.0.0 Not tainted 6.14.0-rc3-syzkaller-00060-g6537cfb395f3 #0
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120
ubsan_epilogue lib/ubsan.c:231 [inline]
__ubsan_handle_out_of_bounds+0x121/0x150 lib/ubsan.c:429
get_nid fs/f2fs/node.h:381 [inline]
f2fs_truncate_inode_blocks+0xa5e/0xf60 fs/f2fs/node.c:1181
f2fs_do_truncate_blocks+0x782/0x1030 fs/f2fs/file.c:808
f2fs_truncate_blocks+0x10d/0x300 fs/f2fs/file.c:836
f2fs_truncate+0x417/0x720 fs/f2fs/file.c:886
f2fs_file_write_iter+0x1bdb/0x2550 fs/f2fs/file.c:5093
aio_write+0x56b/0x7c0 fs/aio.c:1633
io_submit_one+0x8a7/0x18a0 fs/aio.c:2052
__do_sys_io_submit fs/aio.c:2111 [inline]
__se_sys_io_submit+0x171/0x2e0 fs/aio.c:2081
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f238798cde9

index 18446744073709550692 (decimal, unsigned long long)
= 0xfffffffffffffc64 (hexadecimal, unsigned long long)
= -924 (decimal, long long)

In f2fs_truncate_inode_blocks(), UBSAN detects that get_nid() tries to
access .i_nid[-924], it means both offset[0] and level should zero.

The possible case should be in f2fs_do_truncate_blocks(), we try to
truncate inode size to zero, however, dn.ofs_in_node is zero and
dn.node_page is not an inode page, so it fails to truncate inode page,
and then pass zeroed free_from to f2fs_truncate_inode_blocks(), result
in this issue.

if (dn.ofs_in_node || IS_INODE(dn.node_page)) {
f2fs_truncate_data_blocks_range(&dn, count);
free_from += count;
}

I guess the reason why dn.node_page is not an inode page could be: there
are multiple nat entries share the same node block address, once the node
block address was reused, f2fs_get_node_page() may load a non-inode block.

Let's add a sanity check for such condition to avoid out-of-bounds access
issue.