md: simplify get_bitmap_file now that "file" is zeroed.

There is no point assigning '\0' to file->pathname[0] as
file is now zeroed out, so remove that branch and
simplify the code.

[Original patch combined this with the change to use
 kzalloc.  I split the two so that the change to kzalloc
 is easier to backport. - neilb]
Signed-off-by: Benjamin Randazzo <benjamin@randazzo.fr>
Signed-off-by: NeilBrown <neilb@suse.com>

drivers/md/md.c

@@ -5765,16 +5765,16 @@ static int get_bitmap_file(struct mddev *mddev, void __user * arg)
 
 	err = 0;
 	spin_lock(&mddev->lock);
-	/* bitmap disabled, zero the first byte and copy out */
-	if (!mddev->bitmap_info.file)
-		file->pathname[0] = '\0';
-	else if ((ptr = file_path(mddev->bitmap_info.file,
-			       file->pathname, sizeof(file->pathname))),
-		 IS_ERR(ptr))
+	/* bitmap enabled */
+	if (mddev->bitmap_info.file) {
+		ptr = file_path(mddev->bitmap_info.file, file->pathname,
+				sizeof(file->pathname));
+		if (IS_ERR(ptr))
 			err = PTR_ERR(ptr);
 		else
 			memmove(file->pathname, ptr,
 				sizeof(file->pathname)-(ptr-file->pathname));
+	}
 	spin_unlock(&mddev->lock);
 
 	if (err == 0 &&