ffs: Clamp BIO_SPEEDUP length

On 32-bit platforms, the computed size of the BIO_SPEEDUP requested by
softdep_request_cleanup() may be negative when assigned to bp->b_bcount,
which has type "long".

Clamp the size to LONG_MAX. Also convert the unused g_io_speedup() to
use an off_t for the magnitude of the shortage for consistency with
softdep_send_speedup().

Reviewed by: chs, kib
Reported by: pho
Tested by: pho
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D27081