ppp: require callers of ppp_dev_name() to hold RCU

ppp: require callers of ppp_dev_name() to hold RCU

ppp_dev_name() holds the RCU read lock internally to protect pch->ppp.
However, as it returns netdev->name to the caller, the caller should
also hold either RCU or RTNL lock to prevent the netdev from being
freed.

The only two references of the function is in the L2TP driver, both of
which already hold RCU. So remove the internal RCU lock and document
that callers must hold RCU.

Signed-off-by: Qingfang Deng <dqfext@gmail.com>
Reviewed-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20260316092824.479149-1-dqfext@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>