staging: r8188eu: clean up NULL check for rcu pointer

Clean up the NULL check for padapter->pnetdev->rx_handler_data.

The current code calls rcu_dereference while it holds the rcu read lock
and checks the pointer after releasing the lock. An rcu pointer may only be
used between calls to rcu_read_lock and rcu_read_unlock.

Replace the check with rcu_access_pointer. My understanding is that this
function returns the value of the pointer and needs no locking. We can
then check the pointer but we must not dereference it.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20230131090057.241779-1-martin@kaiser.cx


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>