Do not do torn writes to active LDTs.

Care must be taken when updating the active LDT, since parallel
threads might try to load a segment descriptor which is currently
updated. Since the results are undefined, this cannot be ignored by
claiming to be an application race.

Reviewed by: jhb
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D12413