ipsec + ktls: cannot coexists
but instead of tripping the assert in debug kernel, and silently falling
into UB for prod, skip IPSEC processing for KTLS framed packets when
mb_unmapped_to_ext() failed.
Reviewed by: markj
Sponsored by: NVidia networking
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D48265