In commit 96151825 ("rcu: Enable RCU tracepoints by default to aid
in debugging"), CONFIG_RCU_TRACE was made default y (if CONFIG_TREE_RCU=y,
which it is for some of our configs).

That in turn causes CONFIG_TREE_RCU_TRACE to be enabled, which selects
CONFIG_DEBUG_FS. The end result is that CONFIG_DEBUG_FS is forced on,
meaning we don't have to enable it in some of our configs.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>