autofs: fix dev ioctl number range check

The count of miscellaneous device ioctls in fs/autofs4/autofs_i.h is wrong.

The number of ioctls is the difference between AUTOFS_DEV_IOCTL_VERSION_CMD
and AUTOFS_DEV_IOCTL_ISMOUNTPOINT_CMD (14) not the difference between
AUTOFS_IOC_COUNT and 11 (21).

[kusumi.tomohiro@gmail.com: fix typo that made the count macro negative]
 Link: http://lkml.kernel.org/r/20160831033420.9910.16809.stgit@pluto.themaw.net
Link: http://lkml.kernel.org/r/20160812024841.12352.11975.stgit@pluto.themaw.net
Signed-off-by: Ian Kent <raven@themaw.net>
Cc: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/autofs4/autofs_i.h

@@ -20,7 +20,8 @@
 #define AUTOFS_IOC_COUNT     32
 
 #define AUTOFS_DEV_IOCTL_IOC_FIRST	(AUTOFS_DEV_IOCTL_VERSION)
-#define AUTOFS_DEV_IOCTL_IOC_COUNT	(AUTOFS_IOC_COUNT - 11)
+#define AUTOFS_DEV_IOCTL_IOC_COUNT \
+	(AUTOFS_DEV_IOCTL_ISMOUNTPOINT_CMD - AUTOFS_DEV_IOCTL_VERSION_CMD)
 
 #include <linux/kernel.h>
 #include <linux/slab.h>

fs/autofs4/dev-ioctl.c

@@ -642,7 +642,7 @@ static int _autofs_dev_ioctl(unsigned int command,
 	cmd = _IOC_NR(command);
 
 	if (_IOC_TYPE(command) != _IOC_TYPE(AUTOFS_DEV_IOCTL_IOC_FIRST) ||
-	    cmd - cmd_first >= AUTOFS_DEV_IOCTL_IOC_COUNT) {
+	    cmd - cmd_first > AUTOFS_DEV_IOCTL_IOC_COUNT) {
 		return -ENOTTY;
 	}