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/freebsd/zfs/sysctl_os.c

@@ -503,6 +503,24 @@ SYSCTL_UINT(_vfs_zfs_zfetch, OID_AUTO, max_idistance,
 
 /* metaslab.c */
 
+int
+param_set_active_allocator(SYSCTL_HANDLER_ARGS)
+{
+	char buf[16];
+	int rc;
+
+	if (req->newptr == NULL)
+		strlcpy(buf, zfs_active_allocator, sizeof (buf));
+
+	rc = sysctl_handle_string(oidp, buf, sizeof (buf), req);
+	if (rc || req->newptr == NULL)
+		return (rc);
+	if (strcmp(buf, zfs_active_allocator) == 0)
+		return (0);
+
+	return (param_set_active_allocator_common(buf));
+}
+
 /*
  * In pools where the log space map feature is not enabled we touch
  * multiple metaslabs (and their respective space maps) with each

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)
 {