cifs: make compare_sids static

..nothing outside of cifsacl.c calls it. Also fix the incorrect
comment on the function. It returns 0 when they match.

Reviewed-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>

fs/cifs/cifsacl.h

@@ -98,6 +98,4 @@ extern struct key_type cifs_idmap_key_type;
 extern const struct cred *root_cred;
 #endif /* KERNEL */
 
-extern int compare_sids(const struct cifs_sid *, const struct cifs_sid *);
-
 #endif /* _CIFSACL_H */