Avoid doing unnecessary nested TLB invalidations.

Prior to this change the cached value of 'pm_eptgen' was tracked per-vcpu
and per-hostcpu. In the degenerate case where 'N' vcpus were sharing
a single hostcpu this could result in 'N - 1' unnecessary TLB invalidations.
Since an 'invept' invalidates mappings for all VPIDs the first 'invept'
is sufficient.

Fix this by moving the 'eptgen[MAXCPU]' array from 'vmxctx' to 'struct vmx'.

If it is known that an 'invept' is going to be done before entering the
guest then it is safe to skip the 'invvpid'. The stat VPU_INVVPID_SAVED
counts the number of 'invvpid' invalidations that were avoided because
they were subsumed by an 'invept'.

Discussed with:	grehan
libc/net: Fix some issues in inet6_opt_init() (from RFC 3542):

* The RFC says (in section 10.1) that only when extbuf is not NULL,
extlen shall be checked, so don't perform this check when NULL is
passed.

* socklen_t is unsigned, so checking extlen for less than zero is
not needed.

Submitted by:	swildner@dragonflybsd.org
Reviewed by:	Mark Martinec <Mark.Martinec+freebsd@ijs.si>
Reviewed by:	hrs
Obtained by:	DragonFlyBSD