growfs: insist we open a directory when opening fs mount point

This is a simple safety check, since we shouldn't invoke ioctls on fds
without being reasonably sure they are of the right type since ioctls
are overloaded, and we might be tricked hence to execute an operation on
an fd which means something different than what we expect.

src/partition/growfs.c

@@ -225,7 +225,7 @@ static int run(int argc, char *argv[]) {
         if (r < 0)
                 return r;
 
-        mountfd = open(arg_target, O_RDONLY|O_CLOEXEC);
+        mountfd = open(arg_target, O_RDONLY|O_CLOEXEC|O_DIRECTORY);
         if (mountfd < 0)
                 return log_error_errno(errno, "Failed to open \"%s\": %m", arg_target);