XArray: Prevent node leaks in xas_alloc()

XArray: Prevent node leaks in xas_alloc()

In the following situation, we can leak nodes:

do {
	xas_split_alloc();
	xas_lock();
	/* Discover that xas_split() does not need to be called */
	xas_store();
	xas_unlock();
} while (xas_nomem());

The xas_store() is expecting to be using a node allocated by xas_alloc(),
but will use a node allocated by xas_split_alloc() instead.  That
will cause us to leak the remaining nodes which are chained through
node->parent.  Fix this by only popping the top node off the xa_alloc
list instead of removing all the nodes.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>