selftests: net: rds: add getsockopt() conversion test

selftests: net: rds: add getsockopt() conversion test

Add a kselftest that exercises the RDS getsockopt() paths converted to
the getsockopt_iter() / sockopt_t callback:

- RDS_RECVERR and SO_RDS_TRANSPORT, which return their int value through
  copy_to_iter() and report the written length in opt->optlen.

- RDS_INFO_*, which obtains the userspace buffer pages with
  iov_iter_extract_pages() (including a non-zero starting page offset)
  and lets the info producers copy the snapshot in under a spinlock.

Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Allison Henderson <achender@kernel.org>
Link: https://patch.msgid.link/20260608-getsock_more-v3-1-706ecf2ea332@debian.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>