linuxkpi: Fix the shrinker scan target

Use the number of items scanned to control the duration of the shrink
loop. Otherwise, if a consumer like TTM is not able to free the number
of items requested for some reason, the shrinker keeps looping forever.

Reviewed by: manu
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D28224