kernel/manual_su: Define mmap_lock as mmap_sem for old kernel (#495)

kernel/manual_su.h

@@ -5,6 +5,10 @@
 #include <linux/sched.h>
 #include <linux/version.h>
 
+#if LINUX_VERSION_CODE < KERNEL_VERSION(5, 7, 0)
+#define mmap_lock mmap_sem
+#endif
+
 #define KSU_SU_VERIFIED_BIT (1UL << 0)
 #define KSU_TOKEN_LENGTH 32
 #define KSU_TOKEN_ENV_NAME "KSU_AUTH_TOKEN"
@@ -25,4 +29,4 @@ char* ksu_generate_auth_token(void);
 bool ksu_verify_auth_token(const char *token);
 void ksu_cleanup_expired_tokens(void);
 extern bool current_verified;
-#endif
+#endif