ublk: fix build warning on iov_iter_get_pages2

Return type of iov_iter_get_pages2() is ssize_t instead of size_t, so
fix it.

Fixes: 981f95a5 ("ublk: cleanup ublk_copy_user_pages")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Julia Lawall <julia.lawall@inria.fr>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230520151134.459679-1-ming.lei@redhat.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>