MFC r273267:

Add tunable vfs.zfs.space_map_blksz for space map's maximum block size.
MFC r273026:

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>