After commit 3d375d78 ("mm: update callers to use HASH_ZERO flag"),
drop unused pidhash_size in pidhash_init().

Link: http://lkml.kernel.org/r/1500389267-49222-1-git-send-email-wangkefeng.wang@huawei.comSigned-off-by: NKefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: NPavel Tatashin <Pasha.Tatashin@Oracle.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>