LU-6227 vvp: Use range lock for direct IO reads

LU-6227 vvp: Use range lock for direct IO reads

Direct IO does not use the page cache like normal IO, so
concurrent direct IO reads of the same pages are not safe.

As a result, direct IO reads must take the range lock
in ll_file_io_generic, otherwise they will attempt to work
on the same pages and hit assertions like:
(osc_request.c:1219:osc_brw_prep_request())
ASSERTION( i == 0 || pg->off > pg_prev->off ) failed:
i 3 p_c 10 pg ffffea00017a5208 [pri 0 ind 2771] off 16384
prev_pg ffffea00017a51d0 [pri 0 ind 2256] off 16384

Also added a test for dio vs dio reads.

Signed-off-by: Patrick Farrell <paf@cray.com>
Change-Id: I5af524a42891bfa4c52d0fe8d2f7cc94c1ba5c57
Reviewed-on: http://review.whamcloud.com/14385
Tested-by: Jenkins
Reviewed-by: Alexander Boyko <alexander.boyko@seagate.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>