LU-18748 tests: fix sanity/56c with flash devices

LU-18748 tests: fix sanity/56c with flash devices

Fix sanity test_56c to fetch the initial "lfs df" state with "-v"
in case some devices are flash ('f') at the start of the test.

Test-Parameters: trivial
Fixes: 0523ed8625 ("LU-18748 class: print statfs_state for OBD devices")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I3a710c876a0918b20f9bbd7e13cf4b2b1e500c1e
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58543
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>