libc/sys-minix/mount.c: fix overflow

Fix a bug where a filesystem label could overflow the reserved buffer.
This was already possible with 32 bits values, but is more proeminent
with dev_t being 64 bits.

Change-Id: Idc04ed355d1dd92b7a8ce4699de832661a5c4ccd

include/minix/mount.h

@@ -9,6 +9,8 @@
 #define MS_REUSE	0x001	/* Tell RS to try reusing binary from memory */
 #define MS_EXISTING	0x002	/* Tell mount to use already running server */
 
+#define MNT_LABEL_LEN	16	/* Length of fs label including nul */
+
 /* Legacy definitions. */
 #define MNTNAMELEN	16	/* Length of fs type name including nul */
 #define MNTFLAGLEN	64	/* Length of flags string including nul */