locks: print a warning when mount fails due to lack of "mand" support

Since 9e8925b67a ("locks: Allow disabling mandatory locking at compile
time"), attempts to mount filesystems with "-o mand" will fail.
Unfortunately, there is no other indiciation of the reason for the
failure.

Change how the function is defined for better readability. When
CONFIG_MANDATORY_FILE_LOCKING is disabled, printk a warning when
someone attempts to mount with -o mand.

Also, add a blurb to the mandatory-locking.txt file to explain about
the "mand" option, and the behavior one should expect when it is
disabled.

Reported-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jeff Layton <jlayton@kernel.org>

fs/namespace.c

@@ -1643,13 +1643,18 @@ static inline bool may_mount(void)
 	return ns_capable(current->nsproxy->mnt_ns->user_ns, CAP_SYS_ADMIN);
 }
 
+#ifdef	CONFIG_MANDATORY_FILE_LOCKING
 static inline bool may_mandlock(void)
 {
-#ifndef	CONFIG_MANDATORY_FILE_LOCKING
-	return false;
-#endif
 	return capable(CAP_SYS_ADMIN);
 }
+#else
+static inline bool may_mandlock(void)
+{
+	pr_warn("VFS: \"mand\" mount option not supported");
+	return false;
+}
+#endif
 
 /*
  * Now umount can handle mount points as well as block devices.