audit: rename struct node to struct audit_node to prevent future name collisions

audit: rename struct node to struct audit_node to prevent future name collisions

Future work in the powerpc code results in a name collision with the
identified "node" as struct node defined in kernel/audit_tree.c
conflicts with struct node defined in include/linux/node.h (below).
This patch takes the proactive route and renames the audit code's
struct node to audit_node.

	  CC      kernel/audit_tree.o
	kernel/audit_tree.c:33:9: error: redefinition of 'struct node'
	   33 |  struct node {
	      |         ^~~~
	In file included from ./include/linux/cpu.h:17,
                	 from ./include/linux/static_call.h:102,
                	 from ./arch/powerpc/include/asm/machdep.h:10,
                	 from ./arch/powerpc/include/asm/archrandom.h:7,
                	 from ./include/linux/random.h:121,
                	 from ./include/linux/net.h:18,
                	 from ./include/linux/skbuff.h:26,
                	 from kernel/audit.h:11,
                	 from kernel/audit_tree.c:2:
	./include/linux/node.h:84:8: note: originally defined here
	   84 | struct node {
	      |        ^~~~
	make[2]: *** [kernel/audit_tree.o] Error 1

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Richard Guy Briggs <rgb@redhat.com>
[PM: rewrite subj/desc as the build failure is just a RFC patch]
Signed-off-by: Paul Moore <paul@paul-moore.com>