Pass the current/new jail to PR_METHOD_CHECK, which pushes the call
until after the jail is found or created. This requires unlocking the
jail for the call and re-locking it afterward, but that works because
nothing in the jail has been changed yet, and other processes won't
change the important fields as long as allprison_lock remains held.

Keep better track of name vs namelc in kern_jail_set. Name should
always be the hierarchical name (relative to the caller), and namelc
the last component.

PR: 48471
MFC after: 5 days