selftests/namespaces: test for efault

Ensure that put_user() can fail and that namespace cleanup works
correctly.

Link: https://patch.msgid.link/20251109-namespace-6-19-fixes-v1-8-ae8a4ad5a3b3@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

tools/testing/selftests/namespaces/.gitignore

@@ -4,6 +4,7 @@ init_ino_test
 ns_active_ref_test
 listns_test
 listns_permissions_test
+listns_efault_test
 siocgskns_test
 cred_change_test
 stress_test

tools/testing/selftests/namespaces/Makefile

@@ -8,6 +8,7 @@ TEST_GEN_PROGS := nsid_test \
 		  ns_active_ref_test \
 		  listns_test \
 		  listns_permissions_test \
+		  listns_efault_test \
 		  siocgskns_test \
 		  cred_change_test \
 		  stress_test \
@@ -19,6 +20,7 @@ include ../lib.mk
 $(OUTPUT)/ns_active_ref_test: ../filesystems/utils.c
 $(OUTPUT)/listns_test: ../filesystems/utils.c
 $(OUTPUT)/listns_permissions_test: ../filesystems/utils.c
+$(OUTPUT)/listns_efault_test: ../filesystems/utils.c
 $(OUTPUT)/siocgskns_test: ../filesystems/utils.c
 $(OUTPUT)/cred_change_test: ../filesystems/utils.c
 $(OUTPUT)/stress_test: ../filesystems/utils.c