Use buf to send speedup

It turns out there's a problem with using g_io to send the speedup. It leads to
a race when there's a resource shortage when a disk fails.

Instead, send BIO_SPEEDUP via struct buf. This is pretty straight forward,
except we need to transfer the bio_flags from b_ioflags for BIO_SPEEDUP commands
in g_vfs_strategy.

Reviewed by: kirk, chs
Differential Revision: https://reviews.freebsd.org/D23117