LU-15496 tests: fix sanity/398c to use proper OSC name

LU-15496 tests: fix sanity/398c to use proper OSC name

For ppc64le and aarch64 clients, the OSC import instance name does
not have "ffff" at the start, so use the proper device name for this
subtest.

Clean up the rest of test_398c to meet modern test code style.

Test-Parameters: trivial testlist=sanity env=ONLY=398c clientarch=ppc64le clientdistro=el8.8
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: If8c72fa9b13eace009f39daf82454221eba6761b
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55132
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Alex Deiter
Reviewed-by: Oleg Drokin <green@whamcloud.com>