Panic: EHCI and umass

The recent commit really increases up the performance to about 10M a 
second writing to my USB hard drive, but only for smaller file 
transfers.

Large transfers like dumping a filesystem or a tar of a filesystem 
causes the transfer to grind to a halt and eventually panic.  No dump 
is available, here is the transcribed DDB output:

Fatal trap 12: page fault while in kernel mode
fault virtual address   = 0x53425355
fault code              = supervisor read, page not present
instruction pointer     = 0x8:0xc05147d2
stack pointer           = 0x10:0xd4294b6c
frame pointer           = 0x10:0xd4294b8c
code  segment           = base 0x0 limit 0xffff, type 0x1b
                         = DPL0, pres 1, def32 1, gran1
processor eflags        = interrupt enabled, resume, IOPL=0
current process         = 20 (irq10: pcm0 ehci0)
kernel: type 12 trap,code=0

Stopped at usb_allocmem+0x82: cmpl %esi, 0(%eax)

usb_allocmem(c1a50000,d,0,c1a5453c,d429bd4) at usb_allocmem+0x82
ehci_allocm(c1a50000,c1a5453c,d,d429bd8,c057c561) at ehci_allocm_0x27
usbd_transfer(c1a54500,c1a4e500,c1a54800,c1a5485b,d) at usbd_transfer+0x54
umass_setup_transfer(c1a54800,c1a4e50000,c1a5485b,d,0) at umass_setup_transfer+0x51
umass_bbb_state(c1a54700,c1a54800,0,0,c1a5473c) at umass_bbb_state+0x183
usb_transfer_complete(c1a54700,d4294c78,c058c89d,10000,c1a4e480) at usb_transfer_complete+0x111
ehci_idone(c1a54700,0,0,d4294ca4,c04ffe32) at ehci_idone+0xdf
ehci_check_intr(c1a50000,c1a54700,c1a50000,1,d4294cb0) at ehci_check_intr+0x60
ehci_softintr(c1a50000,d4294cd0,c04ffbd4,c1a50000,c06a7155) at ehci_softintr+0x32
usb_schedsoftintr(c1a50000,c06a7155,167,337fba46,c1a70480) at usb_schedsoftintr+0x12
ehci_intr1(c1a500000,d4294d18,c0556a39,c1a50000,0) at ehci_intr1+0xb4
ehci_intr(c1a50000,0,0,0,0) at ehci_intr+0x2f
ithread_loop(c196a880,d4294d48,0,0,c196a880) at ithread_loop+0x199
fork_exit(c05568a0,c19a880,d4284d48) at fork_exit+0x78
fork_trampoline() at fork_trampoline+0x8