lightnvm: add check after mempool allocation

BugLink: http://bugs.launchpad.net/bugs/1531539

The mempool allocation might fail. Make sure to return error when it
does, instead of causing a kernel panic.
Signed-off-by: Javier Gonzalez <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>
(cherry picked from commit 3bfbc6ad)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

drivers/lightnvm/rrpc.c

@@ -287,6 +287,8 @@ static int rrpc_move_valid_pages(struct rrpc *rrpc, struct rrpc_block *rblk)
 	}
 
 	page = mempool_alloc(rrpc->page_pool, GFP_NOIO);
+	if (!page)
+		return -ENOMEM;
 
 	while ((slot = find_first_zero_bit(rblk->invalid_pages,
 					    nr_pgs_per_blk)) < nr_pgs_per_blk) {