mptcp: use tcp_build_frag()

mptcp_push_pending() is called even on orphaned
msk (and orphaned subflows), if there is outstanding
data at close() time.

To cope with the above MPTCP needs to handle explicitly
the allocation failure on xmit. The newly introduced
do_tcp_sendfrag() allows that, just plug it.

We can additionally drop a couple of sanity checks,
duplicate in the TCP code.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>