md/bitmap: fix memory leak of temporary bitmap

Callers of get_bitmap_from_slot() are responsible to free the bitmap.

Suggested-by: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
Signed-off-by: Zhao Heming <heming.zhao@suse.com>
Signed-off-by: Song Liu <songliubraving@fb.com>