mptcp: drop redundant test in move_skbs_to_msk()

Currently we check the msk state to avoid enqueuing new
skbs at msk shutdown time.

Such test is racy - as we can't acquire the msk socket lock -
and useless, as the caller already checked the subflow
field 'disposable', covering the same scenario in a race
free manner - read and updated under the ssk socket lock.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>