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: 50f530e1 ("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