Remove duplicate lz4 implementations

Port illumos change: https://www.illumos.org/issues/11667

Move lz4.c out of zfs tree to opensolaris/common/lz4, adjust it to be
usable from kernel/stand/userland builds, so we can use just one single
source. Add lz4.h to declare lz4_compress() and lz4_decompress().

MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D22037