Handle the corner case in vm_fault_quick_hold_pages().

If supplied length is zero, and user address is invalid, function
might return -1, due to the truncation and rounding of the address.
The callers interpret the situation as EFAULT. Instead of handling
the zero length in caller, filter it in vm_fault_quick_hold_pages().

Sponsored by:	The FreeBSD Foundation
Reviewed by:	alc

sys/vm/vm_fault.c

@@ -1058,6 +1058,8 @@ vm_fault_quick_hold_pages(vm_map_t map, vm_offset_t addr, vm_size_t len,
 	int count;
 	boolean_t pmap_failed;
 
+	if (len == 0)
+		return (0);
 	end = round_page(addr + len);	
 	addr = trunc_page(addr);