timerfd: fix up a memory leak and missing locking

timerfd01 from ltp passes (and some other don't), but none of the tests
crash the kernel.

This is a bare minimum patch to fix up the immediate regression.

Reported by: yasu

(cherry picked from commit 02f534b57f84d6f4f97c337b05b383c8b3aaf18c)

Approved by: re (cperciva@)