On 32-bit platforms, the computed size of the BIO_SPEEDUP submitted by
softdep_request_cleanup() may be negative when assigned to bp->b_bcount,
which has type "long".
Clamp the size to LONG_MAX. We could perhaps limit it to IOSIZE_MAX,
but that would reduce the limit to 2GB on 64-bit platforms, and I'm not
sure if that's desirable.