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

module/os/linux/zfs/spa_misc_os.c

@@ -103,6 +103,18 @@ param_set_slop_shift(const char *buf, zfs_kernel_param_t *kp)
 	return (0);
 }
 
+int
+param_set_active_allocator(const char *val, zfs_kernel_param_t *kp)
+{
+	int error;
+
+	error = -param_set_active_allocator_common(val);
+	if (error == 0)
+		error = param_set_charp(val, kp);
+
+	return (error);
+}
+
 const char *
 spa_history_zone(void)
 {