kern_dup(): Call filecaps_free_prep() in a write section.

filecaps_free_prep() bzeros the capabilities structure and we need to be
careful to synchronize with unlocked readers, which expect a consistent
rights structure.

Reviewed by: kib, mjg
Reported by: syzbot+5f30b507f91ddedded21@syzkaller.appspotmail.com
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D24120