block: Switch from BFQ-v7r3 for 3.0.0 to BFQ-v7r4 for 3.0.0

. BUGFIX. Modified the code so as to be robust against late detection of
  NCQ support for a rotational device.
. BUGFIX. Removed a bug that hindered the correct throughput distribution
  on flash-based devices when not every process had to receive the same
  fraction of the throughput. This fix entailed also a little efficiency
  improvement, because it implied the removal of a short function executed
  in a hot path.
. CODESTYLE IMPROVEMENT: removed quoted strings split across lines.

Signed-off-by: Paolo Valente <paolo.valente@unimore.it>
Reported-by: Steven Barrett <damentz@gmail.com>
Reported-by: Claire Farron <diesal3@googlemail.com>
Signed-off-by: Arianna Avanzini <avanzini.arianna@gmail.com>