seltests: move nsfs into filesystems subfolder

I'm going to be adding new tests for it and it belongs under
filesystem selftests.

Link: https://lore.kernel.org/r/20241213-work-mount-rbtree-lockless-v3-7-6e3cdaf9b280@kernel.org
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

tools/testing/selftests/filesystems/nsfs/Makefile

@@ -3,4 +3,4 @@ TEST_GEN_PROGS := owner pidns
 
 CFLAGS := -Wall -Werror
 
-include ../lib.mk
+include ../../lib.mk