Selectable block allocators

Selectable block allocators

ZFS historically has had several space allocators that were
dynamically selectable.  While these have been retained in
OpenZFS, only a single allocator has been statically compiled
in. This patch compiles all allocators for OpenZFS and provides
a module parameter to allow for manual selection between them.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Edmund Nadolski <edmund.nadolski@ixsystems.com>
Closes #15218