upstream commit: a3dcaf17ee54f1d01d22cc2b22cab0b4f60d78cf
As we want to gradually implement per netns sysctl_rmem and sysctl_wmem
on per protocol basis, add two new fields in struct proto,
and two new helpers : sk_get_wmem0() and sk_get_rmem0()
First user will be TCP. Then UDP and SCTP can be easily converted,
while DECNET probably wont get this support.
Signed-off-by: Hongbo Li <herberthbli@tencent.com>