MFC r215135,215419,215420:

• According to specs for MmAllocateContiguousMemorySpecifyCache() physically contiguous memory with requested restrictions must be allocated.
• Use kmem_alloc_contig() to honour the cache_type variable.
• Fix a panic on i386 for drivers using MmAllocateContiguousMemory() and MmAllocateContiguousMemorySpecifyCache().

Submitted by: Paul B Mahol <onemda at gmail.com>