fs/ntfs3: Use variable length array instead of fixed size

Should fix smatch warning:
	ntfs_set_label() error: __builtin_memcpy() 'uni->name' too small (20 vs 256)

Fixes: 4534a70b

 ("fs/ntfs3: Add headers and misc files")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/r/202401091421.3RJ24Mn3-lkp@intel.com/
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>