x86: x86_msr_op(): MSR_OP_LOCAL: Disable interrupts on atomic ops

On MSR_OP_LOCAL and non-naturally-atomic operations (MSR_OP_ANDNOT and
MSR_OP_OR), there is no guarantee that we are not interrupted between
reading and writing the MSR, and that interruption could actually
perform some operation on that MSR, which would be lost.

Prevent that problem by temporarily disabling interrupts around MSR
manipulation.

Reviewed by: kib
Discussed with: markj
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D54996