umount: ignore missing /proc/swaps

/proc/swaps does not exist when swap is disabled in the kernel.
Just report an empty list of mountpoints to unmount in this case.

src/umount.c

@@ -146,7 +146,7 @@ static int swap_list_get(MountPoint **head) {
         assert(head);
 
         if (!(proc_swaps = fopen("/proc/swaps", "re")))
-                return -errno;
+                return (errno == ENOENT) ? 0 : -errno;
 
         (void) fscanf(proc_swaps, "%*s %*s %*s %*s %*s\n");