landlock: Fix LOG_SUBDOMAINS_OFF inheritance across fork()

In the Linux kernel, the following vulnerability has been resolved:

landlock: Fix LOG_SUBDOMAINS_OFF inheritance across fork()

hook_cred_transfer() only copies the Landlock security blob when the

source credential has a domain. This is inconsistent with

landlock_restrict_self() which can set LOG_SUBDOMAINS_OFF on a

credential without creating a domain (via the ruleset_fd=-1 path): the

field is committed but not preserved across fork() because the child's

prepare_creds() calls hook_cred_transfer() which skips the copy when

domain is NULL.

This breaks the documented use case where a process mutes subdomain logs

before forking sandboxed children: the children lose the muting and

their domains produce unexpected audit records.

Fix this by unconditionally copying the Landlock credential blob.