block, bfq: add Early Queue Merge (EQM) to BFQ-v7r4 for 3.4.0

A set of processes may happen  to  perform interleaved reads, i.e.,
requests whose union would give rise to a  sequential read  pattern. There are two typical  cases: in the first  case,   processes  read fixed-size chunks of data at a fixed distance from each other, while in the second case processes may read variable-size chunks at  variable distances. The latter case occurs for  example with  KVM, which  splits the  I/O generated  by the  guest into multiple chunks,  and lets these chunks  be served by a  pool of cooperating processes,  iteratively assigning  the  next  chunk of  I/O  to  the first available  process. CFQ  uses actual  queue merging  for the  first type of
rocesses, whereas it  uses preemption to get a sequential  read pattern out of the read requests  performed by the second type of  processes. In the end it uses  two different  mechanisms to  achieve the  same goal: boosting the throughput with interleaved I/O.

This patch introduces  Early Queue Merge (EQM), a unified mechanism to get a sequential  read pattern  with
both  types of  processes. The  main idea is checking newly arrived
requests against the next request of the active queue both in case of actual request insert and in case of request merge. By doing so, both the types of processes can be handled by just merging their queues.
EQM is  then simpler and  more compact than the  pair of mechanisms used in CFQ.

Finally, EQM  also preserves the  typical low-latency
properties of BFQ, by properly restoring the weight-raising state of  a queue when it gets back to a non-merged state.

Signed-off-by: Mauro Andreolini <mauro.andreolini@unimore.it>
Signed-off-by: Arianna Avanzini <avanzini.arianna@gmail.com>
Reviewed-by: Paolo Valente <paolo.valente@unimore.it>
Signed-off-by: poondog <markj338@gmail.com>