cam_periph: Return ENXIO when peripheral is invalidated

[imp]

Summary

When the peripheral is invalidated, no further I/O is possible. Signal
this up the stack with ENXIO now that upper layers of the stack
differentiate sometimes. In order for there to be further I/O, and new
open is required for any block device that a future periph might
instantiate for devices at this location that might return or otherwise
become available. The I/O scheduler flushes its I/O with the ENXIO error
for pending I/O that didn't make it to the device, so this makes the two
paths match.

Sponsored by: Netflix

Test Plan

I noticed this when a drive failed under load, and we completed some of the I/O with EIO and some with ENXIO.

[mav]

I think both error codes are correct there, but ENXIO indeed should be more informative here.

[chs]

This makes sense to me.