Introduces two new /sys/block values:

  /sys/block/*/queue/max_hw_sectors_kb
  /sys/block/*/queue/max_sectors_kb

max_hw_sectors_kb is the maximum that the driver can handle and is
readonly.  max_sectors_kb is the current max_sectors value and can be tuned
by root.  PAGE_SIZE granularity is enforced.

It's all locking-safe and all affected layered drivers have been updated as
well.  The patch has been in testing for a couple of weeks already as part
of the voluntary-preempt patches and it works just fine - people use it to
reduce IDE IRQ handling latencies.
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>