Fix off-by-one error in date/time field bit-mask calculation

This became apparent when the the LSB of the day got set (an odd
day). Oops.

This only impacted decoding of date/time, not encoding.

src/smblib/smbdefs.h

@@ -368,7 +368,7 @@ typedef struct {		/* Time with time-zone */
 } when_t;
 
 // We encode Month/Day/Hour/Min/Sec into 26 bits:
-#define SMB_DATE_MK_MASK(width, shift)	(((1 << (width + 1)) - 1) << shift)
+#define SMB_DATE_MK_MASK(width, shift)	(((1 << (width)) - 1) << shift)
 #define SMB_DATE_MON_SHIFT		22
 #define SMB_DATE_MON_MASK		SMB_DATE_MK_MASK(4, SMB_DATE_MON_SHIFT)
 #define SMB_DATE_DAY_SHIFT		17