btree: fix tree corruption in btree_get_prev()

The memory the parameter __key points to is used as an iterator in
btree_get_prev(), so if we save off a bkey() pointer in retry_key and
then assign that to __key, we'll end up corrupting the btree internals
when we do eg

longcpy(__key, bkey(geo, node, i), geo->keylen);

to return the key value. What we should do instead is use longcpy() to
copy the key value that retry_key points to __key.

This can cause a btree to get corrupted by seemingly read-only
operations such as btree_for_each_safe.

[[e-mail]: avoid the double longcpy()]
Signed-off-by: Roland Dreier <[e-mail]>
Acked-by: Joern Engel <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

lib/btree.c

···
 
 	if (head->height == 0)
 		return NULL;
-retry:
 	longcpy(key, __key, geo->keylen);
+retry:
 	dec_key(geo, key);
 
 	node = head->node;
···
 	}
 miss:
 	if (retry_key) {
-		__key = retry_key;
+		longcpy(key, retry_key, geo->keylen);
 		retry_key = NULL;
 		goto retry;
 	}