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
Make sure correct object code is generated at -O0.

Submitted by:	grehan@
Approved by:	so@ (des)
MFC after:	1 month
Eliminate unnecessary checking for M_EXT on mbufs returned by m_getjcl().

Reviewed by:	bz, glebius, yongari
MFC after:	3 days
Sponsored by:	EMC / Isilon Storage Division
Differential Revision:	https://reviews.freebsd.org/D938