MAC/do: Configuration: Fix default values: Remove jail creation method

mac_do_jail_create() would create a default configuration on the
just-created jail, erroneously causing mac_do_jail_set() to then
retrieve it and use it as a model when determining the default values
for not-specified parameters, instead of using the configuration
applicable to the parent jail.

Setting a default configuration in mac_do_jail_create() had been done as
a kind of defensive measure to prevent a created jail not to have
a configuration (effectively making it inherit from an ancestor jail,
which is a security hazard except if explicitly requested).  However,
this measure was never really effective (osd_jail_call(PR_METHOD_CREATE)
in kern_jail_set() calls the PR_PETHOD_CREATE methods in an unspecified
order, and stops at the first error), so we are forced to rely in any
case on the fact that an error in a PR_METHOD_CREATE or PR_METHOD_SET
method leads to stopping the jail creation process (which is the case
today; see kern_jail_set()).

Reviewed by:    bapt
Fixes:          9818224174 ("MAC/do: Executable paths feature (GSoC 2025's final state)")
MFC after:      1 month
Sponsored by:   The FreeBSD Foundation
Pull Request:   https://ron-dev.freebsd.org/FreeBSD/src/pulls/38

sys/security/mac_do/mac_do.c

@@ -1548,16 +1548,6 @@ SYSCTL_PROC(_security_mac_do, OID_AUTO, exec_paths,
 SYSCTL_JAIL_PARAM_STRING(_mac_do, exec_paths, CTLFLAG_RW, MAX_EXEC_PATHS_SIZE,
     "Jail MAC/do executable paths");
 
-static int
-mac_do_jail_create(void *obj, void *data)
-{
-	struct prison *const pr = obj;
-
-	set_default_conf(pr);
-
-	return (0);
-}
-
 static int
 mac_do_jail_get(void *obj, void *data)
 {
@@ -1879,12 +1869,14 @@ mac_do_jail_set(void *obj, void *data)
 /*
  * OSD jail methods.
  *
- * There is no PR_METHOD_REMOVE, as OSD storage is destroyed by the common jail
- * code (see prison_cleanup()), which triggers a run of our dealloc_jail_osd()
- * destructor.
+ * There is no PR_METHOD_REMOVE method, as OSD storage is destroyed by the
+ * common jail code (see prison_cleanup()), which triggers a run of our
+ * dealloc_jail_osd() destructor.  There is neither a PR_METHOD_CREATE as
+ * PR_METHOD_SET is called just after (or the created jail destroyed if some
+ * PR_METHOD_CREATE fails), and our mac_do_jail_set() will ensure a jail is
+ * properly configured.
  */
 static const osd_method_t osd_methods[PR_MAXMETHOD] = {
-	[PR_METHOD_CREATE] = mac_do_jail_create,
 	[PR_METHOD_GET] = mac_do_jail_get,
 	[PR_METHOD_CHECK] = mac_do_jail_check,
 	[PR_METHOD_SET] = mac_do_jail_set,