Make off_linktype, off_nl, and off_nl_nosnap *not* include off_ll.

Make off_linktype, off_nl, and off_nl_nosnap *not* include off_ll.

Make gen_load_llrel() add in off_ll if there isn't a variable-length
radio header, so the offset passed to it is always relative to the
beginning of the link-layer header, not the beginning of the packet.
Don't add in off_ll when calling it.

Those changes centralize the addition of off_ll, so it's only done in
one place (rather than twice, where it was done before, meaning that the
offsets were wrong for the Prism and AVS headers as they had 2*off_ll
rather than just off_ll, as off_ll was added twice).