net/http: Denial of Service vulnerabilities in the HTTP/2 implementation [1.11 backport]

[gopherbot]

@dmitshur requested issue #33606 to be considered for backport to the next 1.11 minor release.

@gopherbot Please backport this to 1.12 and 1.11. This was a security problem.

[dmitshur]

Fixed by e152b01.