bpf: Fix the write filter

[markj]

Summary

A BPF descriptor only has an associated interface descriptor once it is
attached to an interface, e.g., with BIOCSETIF. Avoid dereferencing a
NULL pointer in filt_bpfwrite() if the BPF descriptor is not attached.

Fixes: ded77e0237a8 ("Allow the BPF to be select for write.")
Reported by: [e-mail]

[ae]

LGTM.