lib/test_scanf: Handle n_bits == 0 in random tests

UBSAN reported (via LKP)

[   11.021349][    T1] UBSAN: shift-out-of-bounds in lib/test_scanf.c:275:51
[   11.022782][    T1] shift exponent 32 is too large for 32-bit type 'unsigned int'

When n_bits == 0, the shift is out of range. Switch code to use GENMASK
to handle this case.

Fixes: 50f530e ("lib: test_scanf: Add tests for sscanf number conversion")
Reported-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20210727150132.28920-1-andriy.shevchenko@linux.intel.com

lib/test_scanf.c

@@ -271,7 +271,7 @@ static u32 __init next_test_random(u32 max_bits)
 {
 	u32 n_bits = hweight32(prandom_u32_state(&rnd_state)) % (max_bits + 1);
 
-	return prandom_u32_state(&rnd_state) & (UINT_MAX >> (32 - n_bits));
+	return prandom_u32_state(&rnd_state) & GENMASK(n_bits, 0);
 }
 
 static unsigned long long __init next_test_random_ull(void)
@@ -280,7 +280,7 @@ static unsigned long long __init next_test_random_ull(void)
 	u32 n_bits = (hweight32(rand1) * 3) % 64;
 	u64 val = (u64)prandom_u32_state(&rnd_state) * rand1;
 
-	return val & (ULLONG_MAX >> (64 - n_bits));
+	return val & GENMASK_ULL(n_bits, 0);
 }
 
 #define random_for_type(T)				\