mptcp: remove unneeded lock when listing scheds

mptcp: remove unneeded lock when listing scheds

mptcp_get_available_schedulers() needs to iterate over the schedulers'
list only to read the names: it doesn't modify anything there.

In this case, it is enough to hold the RCU read lock, no need to combine
this with the associated spin lock as it was done since its introduction
in commit 73c900aa3660 ("mptcp: add net.mptcp.available_schedulers").

Suggested-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Geliang Tang <geliang@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20241104-net-next-mptcp-sched-unneeded-lock-v2-1-2ccc1e0c750c@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>