f2fs: fix to set KEEP_SIZE bit in f2fs_zero_range

As Jayashree Mohan reported:

A simple workload to reproduce this would be :
1. create foo
2. Write (8K - 16K)  // foo size = 16K now
3. fsync()
4. falloc zero_range , keep_size (4202496 - 4210688) // foo size must be 16K
5. fdatasync()
Crash now

On recovery, we see that the file size is 4210688 and not 16K, which
violates the semantics of keep_size flag. We have a test case to
reproduce this using CrashMonkey on 4.15 kernel. Try this out by
simply running :
 ./c_harness -f /dev/sda -d /dev/cow_ram0 -t f2fs -e 102400  -P -v
 tests/generic_468_zero.so

The root cause is that we miss to set KEEP_SIZE bit correctly in zero_range
when zeroing block cross EOF with FALLOC_FL_KEEP_SIZE, let's fix this
missing case.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/file.c

@@ -1348,8 +1348,12 @@ static int f2fs_zero_range(struct inode *inode, loff_t offset, loff_t len,
 	}
 
 out:
-	if (!(mode & FALLOC_FL_KEEP_SIZE) && i_size_read(inode) < new_size)
-		f2fs_i_size_write(inode, new_size);
+	if (new_size > i_size_read(inode)) {
+		if (mode & FALLOC_FL_KEEP_SIZE)
+			file_set_keep_isize(inode);
+		else
+			f2fs_i_size_write(inode, new_size);
+	}
 out_sem:
 	up_write(&F2FS_I(inode)->i_mmap_sem);