fs: warn about impending deprecation of mandatory locks

We've had CONFIG_MANDATORY_FILE_LOCKING since 2015 and a lot of distros
have disabled it. Warn the stragglers that still use "-o mand" that
we'll be dropping support for that mount option.

Cc: stable@vger.kernel.org
Signed-off-by: Jeff Layton <jlayton@kernel.org>

fs/namespace.c

@@ -1716,8 +1716,12 @@ static inline bool may_mount(void)
 }
 
 #ifdef	CONFIG_MANDATORY_FILE_LOCKING
-static inline bool may_mandlock(void)
+static bool may_mandlock(void)
 {
+	pr_warn_once("======================================================\n"
+		     "WARNING: the mand mount option is being deprecated and\n"
+		     "         will be removed in v5.15!\n"
+		     "======================================================\n");
 	return capable(CAP_SYS_ADMIN);
 }
 #else