Fix userspace memory leaks found by Clang Static Analzyer

Recently, I have been making a push to fix things that coverity found.
However, I was curious what Clang's static analyzer reported, so I ran
it and found things that coverity had missed.

* contrib/pam_zfs_key/pam_zfs_key.c: If prop_mountpoint is passed more
  than once, we leak memory.
* module/zfs/zcp_get.c: We leak memory on temporary properties in
  userspace.
* tests/zfs-tests/cmd/draid.c: On error from vdev_draid_rand(), we leak
  memory if best_map had been allocated by a prior iteration.
* tests/zfs-tests/cmd/mkfile.c: Memory used by the loop is not freed
  before program termination.

Arguably, these are all minor issues, but if we ignore them, then they
could obscure serious bugs, so we fix them.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #13955

contrib/pam_zfs_key/pam_zfs_key.c

@@ -480,7 +480,8 @@ zfs_key_config_load(pam_handle_t *pamh, zfs_key_config_t *config,
 		} else if (strcmp(argv[c], "nounmount") == 0) {
 			config->unmount_and_unload = 0;
 		} else if (strcmp(argv[c], "prop_mountpoint") == 0) {
-			config->homedir = strdup(entry->pw_dir);
+			if (config->homedir == NULL)
+				config->homedir = strdup(entry->pw_dir);
 		}
 	}
 	return (0);

module/zfs/zcp_get.c

@@ -472,6 +472,7 @@ get_zap_prop(lua_State *state, dsl_dataset_t *ds, zfs_prop_t zfs_prop)
 		/* Fill in temporary value for prop, if applicable */
 		(void) zfs_get_temporary_prop(ds, zfs_prop, &numval, setpoint);
 #else
+		kmem_free(strval, ZAP_MAXVALUELEN);
 		return (luaL_error(state,
 		    "temporary properties only supported in kernel mode",
 		    prop_name));

tests/zfs-tests/cmd/draid.c

@@ -720,8 +720,11 @@ eval_maps(uint64_t children, int passes, uint64_t *map_seed,
 		 */
 		error = alloc_new_map(children, MAP_ROWS_DEFAULT,
 		    vdev_draid_rand(map_seed), &map);
-		if (error)
+		if (error) {
+			if (best_map != NULL)
+				free_map(best_map);
 			return (error);
+		}
 
 		/*
 		 * Consider maps with a lower worst_ratio to be of higher

tests/zfs-tests/cmd/mkfile.c

@@ -276,5 +276,9 @@ main(int argc, char **argv)
 		argv++;
 		argc--;
 	}
+
+	if (buf)
+		free(buf);
+
 	return (errors);
 }