LU-9771 flr: mirror read and write

Support to perform I/O to designated mirror.

Three APIs have been exported for mirror IO:
1. llapi_mirror_read();
2. llapi_mirror_copy();
3. llapi_mirror_{set,clear}().

Test-Parameters: testlist=sanity-flr
Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: Iae1d1edea8e72b423f6f46821594a7d7791ff69b
Reviewed-on: https://review.whamcloud.com/29095
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>