f2fs: fix panic during f2fs_resize_fs()

f2fs_resize_fs() hangs in below callstack with testcase:
- mkfs 16GB image & mount image
- dd 8GB fileA
- dd 8GB fileB
- sync
- rm fileA
- sync
- resize filesystem to 8GB

kernel BUG at segment.c:2484!
Call Trace:
 allocate_segment_by_default+0x92/0xf0 [f2fs]
 f2fs_allocate_data_block+0x44b/0x7e0 [f2fs]
 do_write_page+0x5a/0x110 [f2fs]
 f2fs_outplace_write_data+0x55/0x100 [f2fs]
 f2fs_do_write_data_page+0x392/0x850 [f2fs]
 move_data_page+0x233/0x320 [f2fs]
 do_garbage_collect+0x14d9/0x1660 [f2fs]
 free_segment_range+0x1f7/0x310 [f2fs]
 f2fs_resize_fs+0x118/0x330 [f2fs]
 __f2fs_ioctl+0x487/0x3680 [f2fs]
 __x64_sys_ioctl+0x8e/0xd0
 do_syscall_64+0x33/0x80
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

The root cause is we forgot to check that whether we have enough space
in resized filesystem to store all valid blocks in before-resizing
filesystem, then allocator will run out-of-space during block migration
in free_segment_range().

Fixes: b4b10061ef98 ("f2fs: refactor resize_fs to avoid meta updates in progress")
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/gc.c

@@ -1977,7 +1977,20 @@ int f2fs_resize_fs(struct f2fs_sb_info *sbi, __u64 block_count)
 
 	/* stop CP to protect MAIN_SEC in free_segment_range */
 	f2fs_lock_op(sbi);
+
+	spin_lock(&sbi->stat_lock);
+	if (shrunk_blocks + valid_user_blocks(sbi) +
+		sbi->current_reserved_blocks + sbi->unusable_block_count +
+		F2FS_OPTION(sbi).root_reserved_blocks > sbi->user_block_count)
+		err = -ENOSPC;
+	spin_unlock(&sbi->stat_lock);
+
+	if (err)
+		goto out_unlock;
+
 	err = free_segment_range(sbi, secs, true);
+
+out_unlock:
 	f2fs_unlock_op(sbi);
 	up_write(&sbi->gc_lock);
 	if (err)