Ensure that ZFS ARC free memory checks include cached pages

Also restore kmem_used() check for i386 as it has KVA limits that the raw
page counts above don't consider

PR:		187594
Reviewed by:	peter
X-MFC-With: r270759
Review:	D700
Sponsored by:	Multiplay


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270861 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f