LU-14642 test: add fsx mirror file test mode

LU-14642 test: add fsx mirror file test mode

- add fsx mirror file test mode with "-M" option so that fsx can exert
  its IO to FLR file as well as extend/split/resync the FLR file.

- add sanity-flr test_70b() to test fsx with flrmode.

- fix a bug in "lfs mirror verify" to accomodate max mirror count
  instead of (max - 1) mirrors.

- improve "lfs mirror verify -v" print proper data range of its crc-32
  checksum values.

Test-Parameters: trivial testlist=sanity-flr env=ONLY=100,FSXNUM=10000
Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: Ib55c7b25dcd82fa0b197ad21268b16c82aab5da9
Reviewed-on: https://review.whamcloud.com/43473
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>