sysctl: Merge adjacent CONFIG_TREE_RCU blocks

There are two adjacent sysctl entries protected by the same
CONFIG_TREE_RCU config symbol.  Merge them into a single block to
improve readability.

Use the more common "#ifdef" form while at it.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>