Fix 32-bit maximum volume size

A limit of 1TB exists for zvols on 32-bit systems.  Update the code
to correctly reflect this limitation in a similar manor as the
OpenZFS implementation.

Reviewed-by: Tom Caputi <tcaputi@datto.com>
Reviewed-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #5347

include/sys/zvol.h

@@ -32,6 +32,9 @@
 #define	ZVOL_OBJ		1ULL
 #define	ZVOL_ZAP_OBJ		2ULL
 
+#define	SPEC_MAXOFFSET_T	((1LL << ((NBBY * sizeof (daddr32_t)) + \
+				DEV_BSHIFT - 1)) - 1)
+
 extern void *zvol_tag;
 
 extern void zvol_create_minors(spa_t *spa, const char *name, boolean_t async);