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

MFC after:	2 weeks
Add a complete implementation of MurmurHash3.  Tweak both implementations
so they match the established idiom.  Document them in hash(9).

MFC after:	1 month
MFC with:	r272906
Merge remote-tracking branch 'freebsd/master' into hardened/current/master