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

MFC after:	2 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273267 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
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


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273268 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f