bpf: Fix ifpromisc() locking

[markj]

Summary

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.

[melifaro]

sys/net/bpf.c

1529

Nit: maybe set error to 0 initially to avoid an additional condition?

[markj]

sys/net/bpf.c

1529

This branch is redundant, we can assume that error is already initialized to zero. That's what this handler did before, and others make the same assumption. So I'll just remove this bit.