simple_lmk: Hold an RCU read lock instead of the tasklist read lock

We already check to see if each eligible process isn't already dying, so
an RCU read lock can be used to speed things up instead of holding the
tasklist read lock.

Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
Signed-off-by: Danny Lin <danny@kdrag0n.dev>