Introduce m_get3()

Introduce m_get3() which is similar to m_get2(), but can allocate up to
MJUM16BYTES bytes (m_get2() can only allocate up to MJUMPAGESIZE).

This simplifies the bpf improvement in f13da24715.

Suggested by: glebius
Differential Revision: https://reviews.freebsd.org/D31455