The new net.mptcp.pm_type sysctl determines which path manager will be
used by each newly-created MPTCP socket.

v2: Handle builds without CONFIG_SYSCTL
v3: Clarify logic for type-specific PM init (Geliang Tang and Paolo Abeni)
Acked-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>