Fix build warning from type comparison in min()
    
    On 5.1.0-0.rc7.git2.1.fc31.x86_64, this use of min() results in a
    compiler warning:
    
        ./include/linux/kernel.h:844:29: warning: comparison of distinct pointer types lacks a cast
        ...
        /root/dattobd/src/dattobd.c:2590:21: note: in expansion of macro ‘min’
         2590 |     bytes_to_copy = min(bio_iter_offset(bio, iter) + bio_iter_len(bio, iter) - bvec_off, COW_BLOCK_SIZE - block_off);
              |                     ^~~