Lock around uses of devclass_get_maxunit() and modification of globalIntroduce a new global driver lock to use it in place of Giant. Lock
variables in sysctl handlersaround uses around uses of devclass_get_maxunit().
While here, replace leftover CTLFLAG_NEEDGIANTs with CTLFLAG_MPSAFE.
Sponsored by: The FreeBSD Foundation
MFC after: 2 days