mountpoint-util: introduce mount_propagation_flag_is_valid()

src/basic/mountpoint-util.c

@@ -598,3 +598,7 @@ int mount_propagation_flag_from_string(const char *name, unsigned long *ret) {
                 return -EINVAL;
         return 0;
 }
+
+bool mount_propagation_flag_is_valid(unsigned long flag) {
+        return IN_SET(flag, 0, MS_SHARED, MS_PRIVATE, MS_SLAVE);
+}

src/basic/mountpoint-util.h

@@ -57,3 +57,4 @@ int mount_nofollow(const char *source, const char *target, const char *filesyste
 
 const char *mount_propagation_flag_to_string(unsigned long flags);
 int mount_propagation_flag_from_string(const char *name, unsigned long *ret);
+bool mount_propagation_flag_is_valid(unsigned long flag);