nspawn: ignore EEXIST when mounting tmpfs

commit 79d80fc146 introduced a regression that
prevents mounting a tmpfs if the mount point already exits in the container's
root file system. This commit fixes the problem by ignoring EEXIST.

(cherry picked from commit 6c2d07020f)

src/nspawn/nspawn.c

@@ -818,7 +818,7 @@ static int mount_tmpfs(const char *dest) {
                         return log_oom();
 
                 r = mkdir_label(where, 0755);
-                if (r < 0) {
+                if (r < 0 && errno != EEXIST) {
                         log_error("creating mount point for tmpfs %s failed: %s", where, strerror(-r));
 
                         return r;