include/fs_types: fix unsigned integer overflow

include/fs_types: fix unsigned integer overflow

stats image has (stripe_count * object_size) > UINT32_MAX results following exception:
  ...
  5: (()+0xc3391) [0x7f720e638391]
  6: (librbd::Image::stat(rbd_image_info_t&, unsigned long)+0x46) [0x7f720e5d1fd6]
  7: (rbd::action::info::execute(boost::program_options::variables_map const&)+0x194) [0x7f7218266744]
  8: (rbd::Shell::execute(std::vector<char const*, std::allocator<char const*> > const&)+0x87d) [0x7f721823c1bd]
  9: (main()+0x72) [0x7f721820b4c2]
  ...
  Floating point exception (core dumped)

Signed-off-by: runsisi <runsisi@zte.com.cn>