pam: Fix "buffer overflow" in pam ZTS tests on F38

The pam ZTS tests were reporting a buffer overflow on F38, possibly
due to F38 now setting _FORTIFY_SOURCE=3 by default.  gdb and
valgrind narrowed this down to a snprintf() buffer overflow in
zfs_key_config_modify_session_counter().  I'm not clear why this
particular snprintf() was being flagged as an overflow, but when
I replaced it with an asprintf(), the test passed reliably.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes openzfs#14802 
Closes openzfs#14842

contrib/pam_zfs_key/pam_zfs_key.c

@@ -548,16 +548,11 @@ zfs_key_config_modify_session_counter(pam_handle_t *pamh,
 		    errno);
 		return (-1);
 	}
-	size_t runtime_path_len = strlen(runtime_path);
-	size_t counter_path_len = runtime_path_len + 1 + 10;
-	char *counter_path = malloc(counter_path_len + 1);
-	if (!counter_path) {
+
+	char *counter_path;
+	if (asprintf(&counter_path, "%s/%u", runtime_path, config->uid) == -1)
 		return (-1);
-	}
-	counter_path[0] = 0;
-	strcat(counter_path, runtime_path);
-	snprintf(counter_path + runtime_path_len, counter_path_len, "/%d",
-	    config->uid);
+
 	const int fd = open(counter_path,
 	    O_RDWR | O_CLOEXEC | O_CREAT | O_NOFOLLOW,
 	    S_IRUSR | S_IWUSR);