selftests: add selftest for tcp SO_PEEK_OFF support

selftests: add selftest for tcp SO_PEEK_OFF support

We add a selftest to check that the new feature added in
commit 05ea491641d3 ("tcp: add support for SO_PEEK_OFF socket option")
works correctly.

Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Tested-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Jon Maloy <jmaloy@redhat.com>
Link: https://patch.msgid.link/20240828183752.660267-3-jmaloy@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>