cifs: add a deprecation warning to CIFS_IOC_CHECKUMOUNT ioctl

This was used by an ancient version of umount.cifs and in nowhere else
that I'm aware of. Let's add a warning now and dump it for 3.7.
Signed-off-by: Jeff Layton <jlayton@redhat.com>

fs/cifs/ioctl.c

@@ -51,7 +51,15 @@ long cifs_ioctl(struct file *filep, unsigned int command, unsigned long arg)
 	cifs_sb = CIFS_SB(inode->i_sb);
 
 	switch (command) {
+		static bool warned = false;
 		case CIFS_IOC_CHECKUMOUNT:
+			if (!warned) {
+				warned = true;
+				cERROR(1, "the CIFS_IOC_CHECKMOUNT ioctl will "
+					  "be deprecated in 3.7. Please "
+					  "migrate away from the use of "
+					  "umount.cifs");
+			}
 			cFYI(1, "User unmount attempted");
 			if (cifs_sb->mnt_uid == current_uid())
 				rc = 0;