cfq-iosched: don't delay queue kick for a merged request

"Zhang, Yanmin" <yanmin_zhang@linux.intel.com> reports that commit
b029195d introduced a regression
of about 50% with sequential threaded read workloads. The test
case is:

tiotest -k0 -k1 -k3 -f 80 -t 32

which starts 32 threads each reading a 80MB file. Twiddle the kick
queue logic so that we do start IO immediately, if it appears to be
a fully merged request. We can't really detect that, so just check
if the request is bigger than a page or not. The assumption is that
since single bio issues will first queue a single request with just
one page attached and then later do merges on that, if we already
have more than a page worth of data in the request, then the request
is most likely good to go.

Verified that this doesn't cause a regression with the test case that
commit b029195d

 was fixing. It does not,
we still see maximum sized requests for the queue-then-merge cases.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>