dumpstuff nbytes can't exceed 32 bits

dumpstuff nbytes can't exceed 32 bits

the read returns a 32 bit type; we can't possibly need to write 64 bits.
(ssize_t is 32 bits on macos and so this ends up being a bad shift)

Change-Id: I4d8f99b0c752a2c75ce65b9cfa1b3cb0d484cc7f
Reviewed-on: http://gerrit.openafs.org/2024
Reviewed-by: Russ Allbery <rra@stanford.edu>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>