xfs_scrub: raise media verification IO limits

To avoid starving other threads of disk IO resources, the read-verify
pool limits the size of verification IOs to limit bandwidth consumption,
and it is willing to over-verify some amount of unwritten media to
reduce the number of verification IO requests sent to the device.

However, these limits were set in 2018 when areal densities were lower
and disk bandwidth was more limited.  Increase them now to reduce scan
time on the author's system by 10% in foreground and 50% in background
mode.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>