mptcp: sysctl: avail sched: remove write access

mptcp: sysctl: avail sched: remove write access

'net.mptcp.available_schedulers' sysctl knob is there to list available
schedulers, not to modify this list.

There are then no reasons to give write access to it.

Nothing would have been written anyway, but no errors would have been
returned, which is unexpected.

Fixes: 73c900aa3660 ("mptcp: add net.mptcp.available_schedulers")
Cc: stable@vger.kernel.org
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250108-net-sysctl-current-nsproxy-v1-1-5df34b2083e8@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>