linux-aio: fixes while setting O_DIRECT flag

Linux AIO needs O_DIRECT to be set for effective operation. O_DIRECT
in turn has constraints on when it can work (offset, size alignment)

So use O_DIRECT (unless instructed by application) only when offset
and size alignments match. Else, io_submit() will happen over
non-O_DIRECT fd, effectively blocking till the completion of the IO.

Also fix a multithreading bug where detection/setting of O_DIRECT
for a request was not atomic with io_submit() of that request.

Change-Id: I190017e8bc78217429aff0714dca224cbe6f251d
BUG: 859406
Signed-off-by: Amar Tumballi <amarts@redhat.com>
Reviewed-on: http://review.gluster.org/4006
Tested-by: Amar Tumballi <amarts@redhat.com>
Original-Author: Anand Avati <avati@redhat.com>
Reviewed-on: https://code.engineering.redhat.com/gerrit/61
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Vijay Bellur <vbellur@redhat.com>
Reviewed-on: https://code.engineering.redhat.com/gerrit/1874

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