bpf: Fix BIOCPROMISC locking

BPF might put an interface in promiscuous mode when handling the
BIOCSDLT ioctl. When this happens, a flag is set in the BPF descriptor
so that the old interface can be restored when the BPF descriptor is
destroyed.

The BIOCPROMISC ioctl can also be used to put a BPF descriptor's
interface into promiscuous mode, but there was nothing synchronizing the
flag. Fix this by modifying the ioctl handler to acquire the global BPF
mutex, which is used to synchronize ifpromisc() calls elsewhere in BPF.

Reviewed by: kp, melifaro
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36045