timerfd: convert timerfd_list_lock from sx to mtx

There was no good reason to use the former. This should prevent some
head-scratching by an interested and qualified reader.

(cherry picked from commit f4296cfb409a48de00bfa60e76f686c2b031876f)
(cherry picked from commit a87dddf39e866015341003eeac76d736d993514a)

Approved-by: re (cperciva)