LinuxKPI: allocate current before taking shrinkers lock

This fixes following warnings when shrinkers are invoked first time:

uma_zalloc_debug: zone "lkpicurr" with the following non-sleepable
locks held: exclusive sleep mutex lkpi-shrinker (lkpi-shrinker)

uma_zalloc_debug: zone "lkpimm" with the following non-sleepable locks
held: exclusive sleep mutex lkpi-shrinker (lkpi-shrinker)

Reviewed by: hselasky, manu
Differential revision: https://reviews.freebsd.org/D32066

(cherry picked from commit 2fe9ea5d3ad69d880138d98b2ae8d2c4309eeafa)