kinst: rename t_kinst to t_kinst_tramp

The forthcoming RISC-V and ARM64 ports of kinst introduce a new field
named "t_kinst_curprobe", so "t_kinst" (which points to a trampoline)
becomes a misleading name.

No functional change intended.

Reviewed by: markj
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40507