bcache: add code comment bch_keylist_pop() and bch_keylist_pop_front()

This patch adds simple code comments for bch_keylist_pop() and
bch_keylist_pop_front() in bset.c, to make the code more easier to
be understand.
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/bset.c

@@ -155,6 +155,7 @@ int __bch_keylist_realloc(struct keylist *l, unsigned int u64s)
 	return 0;
 }
 
+/* Pop the top key of keylist by pointing l->top to its previous key */
 struct bkey *bch_keylist_pop(struct keylist *l)
 {
 	struct bkey *k = l->keys;
@@ -168,6 +169,7 @@ struct bkey *bch_keylist_pop(struct keylist *l)
 	return l->top = k;
 }
 
+/* Pop the bottom key of keylist and update l->top_p */
 void bch_keylist_pop_front(struct keylist *l)
 {
 	l->top_p -= bkey_u64s(l->keys);