freebsd: add ifdefs around legacy ioctl support

Require that ZFS_LEGACY_SUPPORT be defined for legacy ioctl support to
be built.  For now, define it in zfs_ioctl_compat.h so support is always
built.  This will allow systems that need never support pre-openzfs
tools a mechanism to remove support at build time.  This code should
be removed once the need for tool compatability is gone.

No functional change at this time.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Brooks Davis <brooks.davis@sri.com>
Closes #14127

lib/libzpool/util.c

@@ -261,7 +261,9 @@ pool_active(void *unused, const char *name, uint64_t guid, boolean_t *isactive)
 	(void) unused, (void) guid;
 	zfs_iocparm_t zp;
 	zfs_cmd_t *zc = NULL;
+#ifdef ZFS_LEGACY_SUPPORT
 	zfs_cmd_legacy_t *zcl = NULL;
+#endif
 	unsigned long request;
 	int ret;
 
@@ -296,6 +298,7 @@ pool_active(void *unused, const char *name, uint64_t guid, boolean_t *isactive)
 		umem_free(zc, sizeof (zfs_cmd_t));
 
 		break;
+#ifdef ZFS_LEGACY_SUPPORT
 	case ZFS_IOCVER_LEGACY:
 		zcl = umem_zalloc(sizeof (zfs_cmd_legacy_t), UMEM_NOFAIL);
 
@@ -311,6 +314,7 @@ pool_active(void *unused, const char *name, uint64_t guid, boolean_t *isactive)
 		umem_free(zcl, sizeof (zfs_cmd_legacy_t));
 
 		break;
+#endif
 	default:
 		fprintf(stderr, "unrecognized zfs ioctl version %d", ver);
 		exit(1);