f2fs: fix to preallocate block only aligned to 4K

In write_begin(), we skip checking dnode block for preallocating block
when whole block needs to be updated since we preallocated its block in
f2fs_preallocate_blocks, for partial updated block, we will still try
to lock its node and do preallocation in write_begin(), so in
f2fs_preallocate_blocks we should not preallocate its block.

But previously, the calculation of preallocating block number is
incorrect, fix it.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
[Jaegeuk Kim: fix a bug]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/data.c

@@ -626,7 +626,12 @@ ssize_t f2fs_preallocate_blocks(struct kiocb *iocb, struct iov_iter *from)
 	ssize_t ret = 0;
 
 	map.m_lblk = F2FS_BLK_ALIGN(iocb->ki_pos);
-	map.m_len = F2FS_BYTES_TO_BLK(iov_iter_count(from));
+	map.m_len = F2FS_BYTES_TO_BLK(iocb->ki_pos + iov_iter_count(from));
+	if (map.m_len > map.m_lblk)
+		map.m_len -= map.m_lblk;
+	else
+		map.m_len = 0;
+
 	map.m_next_pgofs = NULL;
 
 	if (f2fs_encrypted_inode(inode))
@@ -672,6 +677,9 @@ int f2fs_map_blocks(struct inode *inode, struct f2fs_map_blocks *map,
 	bool allocated = false;
 	block_t blkaddr;
 
+	if (!maxblocks)
+		return 0;
+
 	map->m_len = 0;
 	map->m_flags = 0;