f2fs: fix incorrect mapping in ->bmap

Currently, generic_block_bmap is used in f2fs_bmap, its semantics is when
the mapping is been found, return position of target physical block,
otherwise return zero.

But, previously, when there is no mapping info for specified logical block,
f2fs_bmap will map target physical block to a uninitialized variable, which
should be wrong. Fix it.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/data.c

@@ -648,6 +648,8 @@ int f2fs_map_blocks(struct inode *inode, struct f2fs_map_blocks *map,
 	set_new_dnode(&dn, inode, NULL, NULL, 0);
 	err = get_dnode_of_data(&dn, pgofs, mode);
 	if (err) {
+		if (flag == F2FS_GET_BLOCK_BMAP)
+			map->m_pblk = 0;
 		if (err == -ENOENT) {
 			err = 0;
 			if (map->m_next_pgofs)
@@ -683,19 +685,20 @@ int f2fs_map_blocks(struct inode *inode, struct f2fs_map_blocks *map,
 			map->m_flags = F2FS_MAP_NEW;
 			blkaddr = dn.data_blkaddr;
 		} else {
+			if (flag == F2FS_GET_BLOCK_BMAP) {
+				map->m_pblk = 0;
+				goto sync_out;
+			}
 			if (flag == F2FS_GET_BLOCK_FIEMAP &&
 						blkaddr == NULL_ADDR) {
 				if (map->m_next_pgofs)
 					*map->m_next_pgofs = pgofs + 1;
 			}
 			if (flag != F2FS_GET_BLOCK_FIEMAP ||
-						blkaddr != NEW_ADDR) {
-				if (flag == F2FS_GET_BLOCK_BMAP)
-					err = -ENOENT;
+						blkaddr != NEW_ADDR)
 				goto sync_out;
 		}
 	}
-	}
 
 	if (map->m_len == 0) {
 		/* preallocated unwritten block should be mapped for fiemap. */