If userpath is not SHM_ANON, then copy it in early so ktrace(2) can
record it. Without this change, ktrace(2) will attempt to strcpy a
userspace string and trigger a page fault.
Reported by:	syzbot+490b9c2a89f53b1b9779@syzkaller.appspotmail.com
Fixes:		0cd9cde767c3
Approved by:	markj (mentor)
MFC after:	1 month
Sponsored by:	The FreeBSD Foundation