Revert "espintcp: remove encap socket caching to avoid reference leak"

This reverts commit 9cbca301 which is
commit 02836368 upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: Ifec9e4a0d3e7404091dd8bcc60e2cf06815d9c3b
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>