timerfd: compute fflags before calling falloc

While here dodge list locking in timerfd_adjust if empty.

(cherry picked from commit 5eab523053db79b4bd4f926c7d7ac04444d9c1da)

Approved by: re (cperciva@)

(cherry picked from commit fc0c24482f11c403480c5219810a27d8591a8fb6)

Approved-by: re (cperciva)