"panic: malloc(M_WAITOK) in non-sleepable context" when inserting usb stick

r364323, previously I think it was just a warning from witness.  Happens both on boot with stick already inserted, and when inserting the stick in booted system.

Looks like my swap partition is no longer big enough for dump, translating from screen:

umass0 numa-domain 0 on uhub4
umass0:2:0: Attached to scbus2
panic: malloc(M_WAITOK) in non-sleepable context

vpanic()
panic()
malloc()
disk_alloc()
daregister()
cam_periph_alloc()
daasync()
xpt_async_process_dev()
xpt_async_process()
xpt_done_process()
xpt_done_td()
fork_exit()
fork_trampoline()