netback: remove redundant assignment

commit 16ecba2605a3bbd374cd993e532264e2d8de49e5 upstream.

New value for netbk->mmap_pages[pending_idx] is assigned in
xen_netbk_alloc_page(), no need for a second assignment which
exposes internal to the outside world.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>