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.

Lustre-change: https://review.whamcloud.com/43473
Lustre-commit: 90ba8b4ac360b1987178445bd2ccd64f7958d912

Test-Parameters: testlist=sanity-flr env=ONLY=70a,ONLY_REPEAT=10
Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: Ib55c7b25dcd82fa0b197ad21268b16c82aab5da9
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/48834
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>