linux: remove locks around callout_drain in timerfd_close()

The lock around callout_drain() is unnecessary and may cause
deadlock when one closes a timer descriptor during timer execution.

Reviewed By: delphij
Submitted By: ankohuu_outlook.com (Shunchao Hu)
Differential Revision: https://reviews.freebsd.org/D28148

(cherry picked from commit 14c40d2c292deff44be5591add7fdad5f9b371aa)