gf_w64.c: fix integer overflow

Fix for Coverity issue (from Ceph):

CID 1193088 (#1 of 1): Unintentional integer overflow (OVERFLOW_BEFORE_WIDEN)
 overflow_before_widen: Potentially overflowing expression 1 << g_s with type
 int (32 bits, signed) is evaluated using 32-bit arithmetic, and then used in
 a context that expects an expression of type uint64_t (64 bits, unsigned).

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
(cherry picked from commit 1aef869)

src/gf_w64.c

@@ -767,7 +767,7 @@ gf_w64_group_multiply(gf_t *gf, gf_val_64_t a, gf_val_64_t b)
   gd = (struct gf_w64_group_data *) h->private;
   gf_w64_group_set_shift_tables(gd->shift, b, h);
 
-  mask = ((1 << g_s) - 1);
+  mask = (((uint64_t)1 << g_s) - 1);
   top = 0;
   bot = gd->shift[a&mask];
   a >>= g_s;