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

include/sys/metaslab.h

@@ -39,6 +39,7 @@ extern "C" {
 
 
 typedef struct metaslab_ops {
+	const char *msop_name;
 	uint64_t (*msop_alloc)(metaslab_t *, uint64_t);
 } metaslab_ops_t;