uffd: Check for uffd event being PF early

The event received should be checked to be #PF before
accessing its other arguments.

[ Mike:
    Well, looking forward to see non-cooperative userfaultfd patches in kernel
    we should have something like

    static int handle_uffd_enent(struct lazy_pages_info *lpi)
    {
    	read(&msg...);

    	switch (msg.event) {
    	case UFFD_EVENT_PAGEFAULT:
    		handle_pagefault(lpi, msg);
    		break;
    	default:
    		return -1;
    	}
    }

    But since this patch is anyway is a bugfix: <ack>
]

travis-ci: success for uffd: A set of improvements over criu/uffd.c
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Acked-by: Mike Rapoport <rppt@linux.vnet.ibm.com>

criu/uffd.c

@@ -664,6 +664,11 @@ static int handle_user_fault(struct lazy_pages_info *lpi, void *dest)
 		return -1;
 	}
 
+	if (msg.event != UFFD_EVENT_PAGEFAULT) {
+		pr_err("unexpected msg event %u\n", msg.event);
+		return -1;
+	}
+
 	/* Align requested address to the next page boundary */
 	address = msg.arg.pagefault.address & ~(page_size() - 1);
 	pr_debug("msg.arg.pagefault.address 0x%llx\n", address);
@@ -677,11 +682,6 @@ static int handle_user_fault(struct lazy_pages_info *lpi, void *dest)
 	flags = msg.arg.pagefault.flags;
 	pr_debug("msg.arg.pagefault.flags 0x%llx\n", flags);
 
-	if (msg.event != UFFD_EVENT_PAGEFAULT) {
-		pr_err("unexpected msg event %u\n", msg.event);
-		return -1;
-	}
-
 	ret = handle_regular_pages(lpi, dest, address);
 	if (ret < 0) {
 		pr_err("Error during regular page copy\n");