sysusers: add fsync for passwd (#24324)

https://github.com/systemd/systemd/pull/6636 added `fsync()` when
temporary shadow, group, and gshadow files are created, but it was
not added for passwd. As far as I can tell, this seems to have been
an oversight. I'm seeing real world issues where a blank /etc/passwd
file is being created if a machine loses power early in the boot process.

(cherry picked from commit 19193b489841a7bcccda7122ac0849cf6efe59fd)
(cherry picked from commit 9f2f3911539c453037aecd51f875dfd75ed04113)

src/sysusers/sysusers.c

@@ -509,7 +509,7 @@ static int write_temporary_passwd(const char *passwd_path, FILE **tmpfile, char
                         break;
         }
 
-        r = fflush_and_check(passwd);
+        r = fflush_sync_and_check(passwd);
         if (r < 0)
                 return log_debug_errno(r, "Failed to flush %s: %m", passwd_tmp);