block: fix bug with inserting flush requests as sort/merge

With the introduction of the on-stack plugging, we would assume
that any request being inserted was a normal file system request.
As flush/fua requires a special insert mode, this caused problems.

Fix this up by checking for this in flush_plug_list() and use
the appropriate insert mechanism.

Big thanks goes to Markus Tripplesdorf for tirelessly testing
patches, and to Sergey Senozhatsky for helping find the real
issue.

Reported-by: Markus Tripplesdorf <markus@trippelsdorf.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>