cam: use disk_alloc_nowait

[imp]

Summary

CAM drivers are created in a callback in xpt_done_td. This thread is marked
non-sleepable so that callbacks executed here don't block other completions. For
all these periph's register callbacks, use this to allocate the disk. Rearrange
a little so we allocate everything at the top of the function and return the
appropriate error after cleaning upt he allocations.

[rpokala]

Can drivers are created in a callback in xpt_done_td.

"CAM drivers are created in a callback in xpt_done_td." ?

[imp]

In D29163#653097, @rpokala wrote:

Can drivers are created in a callback in xpt_done_td.

"CAM drivers are created in a callback in xpt_done_td." ?

fixed in my local copy and in this description here.

[imp]

On second thought, I'm going to abandon this approach and schedule the cam register routine into a taskqueue so we can sleep. There's no good retry mechanism here, and adding one would make things more complicated. We can switch to waiting for memory to have a more robust system with simpler code. This also matches at least some of the feedback I've received privately (thanks Chuck). I'll post that review when I have it done.