f2fs: don't set FI_PREALLOCATED_ALL for partial write

In f2fs_preallocate_blocks(), if it is partial write in 4KB, it's not
necessary to call f2fs_map_blocks() and set FI_PREALLOCATED_ALL flag.

Cc: Eric Biggers <ebiggers@google.com>
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Author: chaseyu

fs/f2fs/file.c

@@ -4567,7 +4567,8 @@ static int f2fs_preallocate_blocks(struct kiocb *iocb, struct iov_iter *iter,
 	if (map.m_len > map.m_lblk)
 		map.m_len -= map.m_lblk;
 	else
-		map.m_len = 0;
+		return 0;
+
 	map.m_may_create = true;
 	if (dio) {
 		map.m_seg_type = f2fs_rw_hint_to_seg_type(inode->i_write_hint);