netgraph/ng_tag: permit variable length data

ng_tag(4) operate on arbitrary data of mbuf_tags(9). Those structures
are padded to the next multiple of the alignment by the compiler.
Hence a valid argument has be at most as long as the data received.

PR: 241462
Reviewed by: kp
Approved by: kp (mentor)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D22140