Bug 29194

When reading from a raw device, the userspace buffer is mapped into kernel
virtal address space.  To ensure that the pages are writable, a byte is read
from each relevant page and written back in vm_fault_quick to force a
copy-on-write if needed.  Unless the userspace buffer is page aligned, the
first writeback is outside the userspace buffer and might cause modification to
that byte by another CPU or DMA to be lost.

Fix: Don't write to 'unrelated' memory to force a copy-on-write.

Use the buffer start address instead of the page start address as the first
argument to vm_fault_quick if the page start address is below the buffer start
address.
How-To-Repeat: 
Perform multiple simultaneous small (2 KB) read operations from raw device to
memory in the same address space where pairs of userspace buffers share a page.