Type validating smr protected pointer accessors.

This API is intended to provide some measure of safety with SMR
protected pointers. A struct wrapper provides type checking and
a guarantee that all access is mediated by the API unless abused. All
modifying functions take an assert as an argument to guarantee that
the required synchronization is present.

Reviewed by: kib, markj, mjg
Differential Revision: https://reviews.freebsd.org/D23711