ext4: let ext4_discard_partial_buffers handle unaligned range correctly

As comment says, we should handle unaligned range rather than aligned
one.  This fixes a bug found by running xfstests #91.

Signed-off-by: Yongqiang Yang <xiaoqiangnk@gmail.com>
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 5fcef98..de05e86 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3206,8 +3206,8 @@
 		 * to be updated with the contents of the block before
 		 * we write the zeros on top of it.
 		 */
-		if (!(from & (blocksize - 1)) ||
-		    !((from + length) & (blocksize - 1))) {
+		if ((from & (blocksize - 1)) ||
+		    ((from + length) & (blocksize - 1))) {
 			create_empty_buffers(page, blocksize, 0);
 		} else {
 			/*