f2fs: fix to avoid use f2fs_bug_on() in f2fs_new_node_page()

As Dipanjan Das <mail.dipanjan.das@gmail.com> reported, syzkaller
found a f2fs bug as below:

RIP: 0010:f2fs_new_node_page+0x19ac/0x1fc0 fs/f2fs/node.c:1295
Call Trace:
 write_all_xattrs fs/f2fs/xattr.c:487 [inline]
 __f2fs_setxattr+0xe76/0x2e10 fs/f2fs/xattr.c:743
 f2fs_setxattr+0x233/0xab0 fs/f2fs/xattr.c:790
 f2fs_xattr_generic_set+0x133/0x170 fs/f2fs/xattr.c:86
 __vfs_setxattr+0x115/0x180 fs/xattr.c:182
 __vfs_setxattr_noperm+0x125/0x5f0 fs/xattr.c:216
 __vfs_setxattr_locked+0x1cf/0x260 fs/xattr.c:277
 vfs_setxattr+0x13f/0x330 fs/xattr.c:303
 setxattr+0x146/0x160 fs/xattr.c:611
 path_setxattr+0x1a7/0x1d0 fs/xattr.c:630
 __do_sys_lsetxattr fs/xattr.c:653 [inline]
 __se_sys_lsetxattr fs/xattr.c:649 [inline]
 __x64_sys_lsetxattr+0xbd/0x150 fs/xattr.c:649
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x46/0xb0

NAT entry and nat bitmap can be inconsistent, e.g. one nid is free
in nat bitmap, and blkaddr in its NAT entry is not NULL_ADDR, it
may trigger BUG_ON() in f2fs_new_node_page(), fix it.
Reported-by: Dipanjan Das <mail.dipanjan.das@gmail.com>
Signed-off-by: Chao Yu <chao.yu@oppo.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/node.c

@@ -1292,7 +1292,11 @@ struct page *f2fs_new_node_page(struct dnode_of_data *dn, unsigned int ofs)
 		dec_valid_node_count(sbi, dn->inode, !ofs);
 		goto fail;
 	}
-	f2fs_bug_on(sbi, new_ni.blk_addr != NULL_ADDR);
+	if (unlikely(new_ni.blk_addr != NULL_ADDR)) {
+		err = -EFSCORRUPTED;
+		set_sbi_flag(sbi, SBI_NEED_FSCK);
+		goto fail;
+	}
 #endif
 	new_ni.nid = dn->nid;
 	new_ni.ino = dn->inode->i_ino;