selftests/namespace: add stress test

Stress tests for namespace active reference counting.

These tests validate that the active reference counting system can
handle high load scenarios including rapid namespace
creation/destruction, large numbers of concurrent namespaces, and
various edge cases under stress.

Link: https://patch.msgid.link/20251029-work-namespace-nstree-listns-v4-71-2e6f823ebdc0@kernel.org
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

tools/testing/selftests/namespaces/.gitignore

@@ -6,3 +6,4 @@ listns_test
 listns_permissions_test
 siocgskns_test
 cred_change_test
+stress_test

tools/testing/selftests/namespaces/Makefile

@@ -9,7 +9,8 @@ TEST_GEN_PROGS := nsid_test \
 		  listns_test \
 		  listns_permissions_test \
 		  siocgskns_test \
-		  cred_change_test
+		  cred_change_test \
+		  stress_test
 
 include ../lib.mk
 
@@ -18,4 +19,5 @@ $(OUTPUT)/listns_test: ../filesystems/utils.c
 $(OUTPUT)/listns_permissions_test: ../filesystems/utils.c
 $(OUTPUT)/siocgskns_test: ../filesystems/utils.c
 $(OUTPUT)/cred_change_test: ../filesystems/utils.c
+$(OUTPUT)/stress_test: ../filesystems/utils.c