bcache: Add bch_keylist_init_single()

This will potentially save us an allocation when we've got inode/dirent bkeys
that don't fit in the keylist's inline keys.

Signed-off-by: Kent Overstreet <kmo@daterainc.com>
diff --git a/drivers/md/bcache/bset.h b/drivers/md/bcache/bset.h
index 003260f..5f6728d 100644
--- a/drivers/md/bcache/bset.h
+++ b/drivers/md/bcache/bset.h
@@ -478,6 +478,12 @@
 	l->top_p = l->keys_p = l->inline_keys;
 }
 
+static inline void bch_keylist_init_single(struct keylist *l, struct bkey *k)
+{
+	l->keys = k;
+	l->top = bkey_next(k);
+}
+
 static inline void bch_keylist_push(struct keylist *l)
 {
 	l->top = bkey_next(l->top);