ext4: simplify calculation of blocks to free on error

In ext4_ext_map_blocks(), if we have successfully allocated the data
blocks, but then run into trouble inserting the extent into the extent
tree, most likely due to an ENOSPC condition, determine the arguments
to ext4_free_blocks() in a simpler way which is easier to prove to be
correct.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/extents.c

@@ -4261,8 +4261,8 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
 		/* not a good idea to call discard here directly,
 		 * but otherwise we'd need to call it every free() */
 		ext4_discard_preallocations(inode);
-		ext4_free_blocks(handle, inode, NULL, ext4_ext_pblock(&newex),
-				 ext4_ext_get_actual_len(&newex), fb_flags);
+		ext4_free_blocks(handle, inode, NULL, newblock,
+				 EXT4_C2B(sbi, allocated_clusters), fb_flags);
 		goto out2;
 	}