try to satisfy the entire read
try to satisfy the entire read

not strictly necessary, but purely for the benefits of not having to
deal with bug reports from code that doesn't properly check the return
value of read(). iirc, this doesn't normally happen for
ondisk-filesystems and can only happen in NFS in certain mount modes

fixes #177

kahing/goofys,kahing/goofys,kahing/goofys