add a knob that disables detection of write protected disks

[avg]

Summary

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.

Test Plan

Tested disk probing with WP detection on and off.

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While here, I also fixed setting of the next state in the case of malloc failure in DA_STATE_PROBE_WP.

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correct description of the new sysctl

[imp]

sys/cam/scsi/scsi_da.c

3343

wouldn't it be better to go directly to PROBE_RC* rather than have this useless trip through STATE_PROBE_WP?

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sys/cam/scsi/scsi_da.c

3343

That's certainly another possibility to do it, but I am not sure if it's really better.
With respect to run-time execution the difference is negligible, in my opinion.
With respect to code organization da_disable_wp_detection would have to be checked in two places, daregister and dareprobe, instead of one. But that's a pretty minor difference as well.

So, I don't have a preference. I did it this way, but I can re-do it the other way.

3347

I wonder if there should be an inline function for choosing between DA_STATE_PROBE_RC16 and DA_STATE_PROBE_RC, given that the check is going to be repeated 3 or 4 times.