add a knob that disables detection of write protected disks

add a knob that disables detection of write protected disks

It has been reported that on some systems (with real hardware passed
through to a virtual machine) the WP detection causes USB disk probing
failures.

While here, also fix the selection of the next state in the case
of malloc failure in DA_STATE_PROBE_WP.  It was DA_STATE_PROBE_RC
unconditionally even when it should have been DA_STATE_PROBE_RC16.

PR:		225794
Reported by:	David Boyd <David.Boyd49@twc.com>
MFC after:	3 weeks
Differential Revision: https://reviews.freebsd.org/D18496