nbd/server: Kill pointless shadowed variable

lgtm.com pointed out that commit 678ba275 introduced a shadowed
declaration of local variable 'bs'; thankfully, the inner 'bs'
obtained by 'blk_bs(blk)' matches the outer one given that we had
'blk_insert_bs(blk, bs, errp)' a few lines earlier, and there are
no later uses of 'bs' beyond the scope of the 'if (bitmap)' to
care if we change the value stored in 'bs' while traveling the
backing chain to find a bitmap.  So simply get rid of the extra
declaration.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190207191357.6665-1-eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>