System-wide speculative prefetch limit.

With some pathological access patterns it is possible to make ZFS
accumulate almost unlimited amount of speculative prefetch ZIOs.
Combined with linear ABD allocations in RAIDZ code, it appears to
be possible to exhaust system KVA, triggering kernel panic.

Address this by introducing a system-wide counter of active prefetch
requests and blocking prefetch distance doubling per stream hits if
the number of active requests is higher that ~6% of ARC size.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #14516