x86/fpu: Cleanup variable shadowing

Addresses: warning: Local variable 'mask' shadows outer variable

Remove extra variable declaration and switch the bit mask assignment to use
BIT_ULL() while at it.

Fixes: 522e9274

 ("x86/fpu: Deduplicate copy_uabi_from_user/kernel_to_xstate()")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/lkml/202204262032.jFYKit5j-lkp@intel.com