Partially revert r248770.

Under geom(4) nvme_ns_bio_process() is on the path where sleep
is prohibited as g_io_shedule_down() calls THREAD_NO_SLEEPNG()
before geom->start().

Reviewed By: imp
Differential Revision: https://reviews.freebsd.org/D29539

(cherry picked from commit a78109d5db87b08785a822770e2e4fdb15f921b6)