uvideo: replace contigmalloc with OBJT_PHYS vm_object for mmap buffer

[bapt]

Summary

Allocate the mmap buffer via phys_pager_allocate() and map it into
kernel space with vm_map_find()/vm_map_wire(), instead of a custom
cdev_pager backed by contigmalloc. phys_pager_allocate() is required
over a bare vm_object_allocate(OBJT_PHYS) to initialise un_pager.phys.ops,
otherwise phys_pager_getpages() NULL-derefs during vm_map_wire().

Reported by: markj

[markj]

sys/dev/usb/video/uvideo.c

3588

I believe this vm_object_deallocate() is wrong and should be removed. The vm_map_find() call above consumes a reference to obj, and then the vm_map_remove() call releases that reference. See the comment above vm_map_find().

I believe uvideo_vs_free_frame() has the same problem, it calls vm_map_remove(), which releases the original reference, then calls vm_object_deallocate() again.

[bapt]

address @markj's comment about deallocate

[markj]

Seems fine. There are other problems in the cdev methods, but this change on its own looks ok.

sys/dev/usb/video/uvideo.c

3115

Can this addition overflow?