new-bus: Use M_WAITOK in more places

M_NOWAIT is now only used for devclass_get_devices,
devclass_get_drivers, and device_get_children. Other functions are
generally called from sleepable contexts such as module event
handlers, attach/detach routines, and the ioctl handler for
/dev/devctl2.

Suggested by: mjg (devclass_add_device)
Differential Revision: https://reviews.freebsd.org/D49274