storvsc: get rid of bounce buffer

Storvsc driver needs to ensure there are no 'holes' in the presented
sg list (all segments in the middle of the list need to be of PAGE_SIZE).
When a hole is detected storvsc driver creates a 'bounce sgl' without
holes and copies data over with copy_{to,from}_bounce_buffer() functions.
Setting virt_boundary_mask to PAGE_SIZE - 1 guarantees we'll never see
such holes so we can significantly simplify the driver. This is also
supposed to bring us some performance improvement for certain workloads
as we eliminate copying.

Reported-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: K. Y. Srinivasan <kys@microsoft.com>
Tested-by: Alex Ng <alexng@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: James Bottomley <JBottomley@Odin.com>