Will be needed by the next patch, as MPTCP needs to handle
directly the error/memory-allocation-needed path.

No functional changes intended.

Additionally let MPTCP code access the tcp_remove_empty_skb()
helper.
Signed-off-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>