md-bitmap: small cleanups

In md_bitmap_unplug, bitmap->storage.filemap is double checked.

In md_bitmap_daemon_work, bitmap->storage.filemap should be checked
before reference.
Signed-off-by: Zhiqiang Liu <liuzhiqiang26@huawei.com>
Signed-off-by: Song Liu <songliubraving@fb.com>

drivers/md/md-bitmap.c

@@ -1019,8 +1019,6 @@ void md_bitmap_unplug(struct bitmap *bitmap)
 	/* look at each page to see if there are any set bits that need to be
 	 * flushed out to disk */
 	for (i = 0; i < bitmap->storage.file_pages; i++) {
-		if (!bitmap->storage.filemap)
-			return;
 		dirty = test_and_clear_page_attr(bitmap, i, BITMAP_PAGE_DIRTY);
 		need_write = test_and_clear_page_attr(bitmap, i,
 						      BITMAP_PAGE_NEEDWRITE);
@@ -1338,7 +1336,8 @@ void md_bitmap_daemon_work(struct mddev *mddev)
 				   BITMAP_PAGE_DIRTY))
 			/* bitmap_unplug will handle the rest */
 			break;
-		if (test_and_clear_page_attr(bitmap, j,
+		if (bitmap->storage.filemap &&
+		    test_and_clear_page_attr(bitmap, j,
 					     BITMAP_PAGE_NEEDWRITE)) {
 			write_page(bitmap, bitmap->storage.filemap[j], 0);
 		}