kcm: Destroy mutex in kcm_exit_net()

kcm_exit_net() should call mutex_destroy() on knet->mutex. This is especially
needed if CONFIG_DEBUG_MUTEXES is enabled.

Fixes: ab7ac4eb ("kcm: Kernel Connection Multiplexor module")
Signed-off-by: Shigeru Yoshida <syoshida@redhat.com>
Link: https://lore.kernel.org/r/20230902170708.1727999-1-syoshida@redhat.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>