Fix a memory leak for the BIOCSETWF ioctl on kernels with the BPF_JITTER
option.

The BPF code was creating a compiled filter in the common filter-creation
path. However, BPF only uses compiled filters in the read direction.
When creating a write filter, the common filter-creation code was
creating an unneeded write filter and leaking the memory used for that.

MFC after: 2 weeks
Sponsored by: Netflix