Update flashcache_reclaim.c

The function of flashcache_reclaim_demote_block() is to swap this hot block with the MRU block in the warm queue.

And the MRU block in the warm queue is related to warmlist_lru_tail,so

the code:
warm_block = cache_set->warmlist_lru_head; 

should be:
warm_block = cache_set->warmlist_lru_tail;