Add a tunable for arc_shrink_shift (vfs.zfs.arc_shrink_shift) that
controls how much fraction, 1/2^arc_shrink_shift, should be reclaimed
when there is memory pressure.

Submitted by:	Richard Kojedzinszky <krichy at tvnetwork.hu>
MFC after:	2 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273026 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f