Introduce the function kmem_alloc_attr(), which allocates kernel virtual

Introduce the function kmem_alloc_attr(), which allocates kernel virtual
memory with the specified physical attributes.  In particular, like
kmem_alloc_contig(), the caller can specify the physical address range
from which the physical pages are allocated and the memory attributes
(i.e., cache behavior) for these physical pages.  However, in contrast to
kmem_alloc_contig() or contigmalloc(), the physical pages that are
allocated by kmem_alloc_attr() are not necessarily physically contiguous.
This function is needed by DRM and VirtualBox.

Correct an error in the prototype for kmem_malloc().  The third argument
had the wrong type.

Tested by:	rnoland
MFC after:	3 days