re: kern/46648: disconnecting usb mouse crashes system

[matthew green <mrg%eterna.com.au@localhost>]

hmmmm, this problem looks pretty annoying, and might affect many
devices.  the problem seems to be that:

        - device was pulled out with active transfers

        - the handling of an error in the transfer callback function
        is to call usbd_clear_endpoint_stall_async(), which then
        sends a clear endpoint transfer

        - adapter transfer list is empty, causing kmem_alloc call,
        triggering the assertion since this is happening from the
        softintr thread

this idiom is used in many of the USB drivers, so a solution that
does not require each one changing seems appropriate.

i'm not sure yet exactly the right answer.  it's clear to me (and
has been or a while, since before usbmp) that the USB softintr
does way too much work.


.mrg.