ZTS: FreeBSD does have a l2arc.trim_ahead tunable

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10633

tests/zfs-tests/include/tunables.cfg

@@ -39,7 +39,7 @@ LUA_MAX_MEMLIMIT		lua.max_memlimit		zfs_lua_max_memlimit
 L2ARC_NOPREFETCH		l2arc.noprefetch		l2arc_noprefetch
 L2ARC_REBUILD_BLOCKS_MIN_L2SIZE	l2arc.rebuild_blocks_min_l2size	l2arc_rebuild_blocks_min_l2size
 L2ARC_REBUILD_ENABLED		l2arc.rebuild_enabled		l2arc_rebuild_enabled
-L2ARC_TRIM_AHEAD		UNSUPPORTED			l2arc_trim_ahead
+L2ARC_TRIM_AHEAD		l2arc.trim_ahead		l2arc_trim_ahead
 L2ARC_WRITE_BOOST		l2arc.write_boost		l2arc_write_boost
 L2ARC_WRITE_MAX			l2arc.write_max			l2arc_write_max
 LIVELIST_CONDENSE_NEW_ALLOC	livelist.condense.new_alloc	zfs_livelist_condense_new_alloc