f2fs: fix to avoid panic in is_alive() if metadata is inconsistent

As report by Wenqing Liu in bugzilla:

https://bugzilla.kernel.org/show_bug.cgi?id=215231

If we enable CONFIG_F2FS_CHECK_FS config, and with fuzzed image attached
in above link, we will encounter panic when executing below script:

1. mkdir mnt
2. mount -t f2fs tmp1.img mnt
3. touch tmp

F2FS-fs (loop11): mismatched blkaddr 5765 (source_blkaddr 1) in seg 3
kernel BUG at fs/f2fs/gc.c:1042!
 do_garbage_collect+0x90f/0xa80 [f2fs]
 f2fs_gc+0x294/0x12a0 [f2fs]
 f2fs_balance_fs+0x2c5/0x7d0 [f2fs]
 f2fs_create+0x239/0xd90 [f2fs]
 lookup_open+0x45e/0xa90
 open_last_lookups+0x203/0x670
 path_openat+0xae/0x490
 do_filp_open+0xbc/0x160
 do_sys_openat2+0x2f1/0x500
 do_sys_open+0x5e/0xa0
 __x64_sys_openat+0x28/0x40

Previously, f2fs tries to catch data inconcistency exception in between
SSA and SIT table during GC, however once the exception is caught, it will
call f2fs_bug_on to hang kernel, it's not needed, instead, let's set
SBI_NEED_FSCK flag and skip migrating current block.

Fixes: bbf9f7d9 ("f2fs: Fix indefinite loop in f2fs_gc()")
Signed-off-by: NChao Yu <chao@kernel.org>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/gc.c

@@ -1039,7 +1039,7 @@ static bool is_alive(struct f2fs_sb_info *sbi, struct f2fs_summary *sum,
 			if (!test_and_set_bit(segno, SIT_I(sbi)->invalid_segmap)) {
 				f2fs_err(sbi, "mismatched blkaddr %u (source_blkaddr %u) in seg %u",
 					 blkaddr, source_blkaddr, segno);
-				f2fs_bug_on(sbi, 1);
+				set_sbi_flag(sbi, SBI_NEED_FSCK);
 			}
 		}
 #endif