cred: conditionally declare groups-related functions

The groups-related functions declared in include/linux/cred.h are
defined in kernel/groups.c, which is compiled only when
CONFIG_MULTIUSER=y. Move all these function declarations under #ifdef
CONFIG_MULTIUSER to help avoid accidental usage in contexts where
CONFIG_MULTIUSER might be disabled.

This patch also adds a fallback for groups_search(). Currently this
function is only called from kernel/groups.c itself and
security/keys/permissions.c, where the call is (by coincidence)
optimized away in case CONFIG_MULTIUSER=n. However, the audit subsystem
(which does not depend on CONFIG_MULTIUSER) calls this function in
-next, so the fallback will be needed to avoid compilation errors or
ugly workarounds.

See also:
https://lkml.org/lkml/2018/6/20/670
https://git.kernel.org/pub/scm/linux/kernel/git/pcmoore/audit.git/commit/?h=next&id=af85d1772e31fed34165a1b3decef340cf4080c0

Reported-by: Randy Dunlap <[e-mail]>
Tested-by: Randy Dunlap <[e-mail]>
Signed-off-by: Ondrej Mosnacek <[e-mail]>
Signed-off-by: Paul Moore <[e-mail]>

include/linux/cred.h

···
 
 extern int in_group_p(kgid_t);
 extern int in_egroup_p(kgid_t);
+extern int groups_search(const struct group_info *, kgid_t);
+
+extern int set_current_groups(struct group_info *);
+extern void set_groups(struct cred *, struct group_info *);
+extern bool may_setgroups(void);
+extern void groups_sort(struct group_info *);
 #else
 static inline void groups_free(struct group_info *group_info)
 {
···
 {
         return 1;
 }
+static inline int groups_search(const struct group_info *group_info, kgid_t grp)
+{
+	return 1;
+}
 #endif
-extern int set_current_groups(struct group_info *);
-extern void set_groups(struct cred *, struct group_info *);
-extern int groups_search(const struct group_info *, kgid_t);
-extern bool may_setgroups(void);
-extern void groups_sort(struct group_info *);
 
 /*
  * The security context of a task