ktrace: change AST handler to require AST flag set

When it was inline it made sense to depend on the existing nested check
in KTRUSERRET() rather than adding a new td_flags flag. However, since
we now have a TDA_KTRACE flag anyway, we might as well check it and
avoid the call.

Suggested by: jhb
Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D35888