mptcp: init: protect sched with rcu_read_lock

In the Linux kernel, the following vulnerability has been resolved:

mptcp: init: protect sched with rcu_read_lock

Enabling CONFIG_PROVE_RCU_LIST with its dependence CONFIG_RCU_EXPERT
creates this splat when an MPTCP socket is created:

=============================
WARNING: suspicious RCU usage
6.12.0-rc2+ #11 Not tainted
-----------------------------
net/mptcp/sched.c:44 RCU-list traversed in non-reader section!!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
no locks held by mptcp_connect/176.

stack backtrace:
CPU: 0 UID: 0 PID: 176 Comm: mptcp_connect Not tainted 6.12.0-rc2+ #11
Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
Call Trace:
<TASK>
dump_stack_lvl (lib/dump_stack.c:123)
lockdep_rcu_suspicious (kernel/locking/lockdep.c:6822)
mptcp_sched_find (net/mptcp/sched.c:44 (discriminator 7))
mptcp_init_sock (net/mptcp/protocol.c:2867 (discriminator 1))
? sock_init_data_uid (arch/x86/include/asm/atomic.h:28)
inet_create.part.0.constprop.0 (net/ipv4/af_inet.c:386)
? __sock_create (include/linux/rcupdate.h:347 (discriminator 1))
__sock_create (net/socket.c:1576)
__sys_socket (net/socket.c:1671)
? __pfx___sys_socket (net/socket.c:1712)
? do_user_addr_fault (arch/x86/mm/fault.c:1419 (discriminator 1))
__x64_sys_socket (net/socket.c:1728)
do_syscall_64 (arch/x86/entry/common.c:52 (discriminator 1))
entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)

That's because when the socket is initialised, rcu_read_lock() is not
used despite the explicit comment written above the declaration of
mptcp_sched_find() in sched.c. Adding the missing lock/unlock avoids the
warning.