xen-blkfront: switch kcalloc to kvcalloc for large array allocation

commit 1d5c76e66433382a1e170d1d5845bb0fed7467aa upstream.

There's no reason to request physically contiguous memory for those
allocations.

[boris: added CC to stable]

Cc: stable@vger.kernel.org
Reported-by: Ian Jackson <ian.jackson@citrix.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>