Staging: hv: Zero out the request structure on allocation

As part of the cleanup of the I/O request structure, allocate them zeroed
out. This fixes a bug that was introduced in the patch-set titled:
Staging: hv: Cleanup-storage-drivers-phase-II.
A corrected version of this patch-set was sent out earlier, however
the earlier (buggy) version of this patch-set was already applied.
This patch fixes the problem.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Hank Janssen <hjanssen@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>