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.

Lustre-change: https://review.whamcloud.com/55132
Lustre-commit: b1b57bcadeeb5a87ac75387c4aa4ae084e1a27e0

LU-15496 tests: add debugging to sanity/398c

Dump the rpc_stats to help understand why the test is failing.

Lustre-change: https://review.whamcloud.com/53462
Lustre-commit: 304ca31e2aa15c576e468a86e45d8817c8eca391

Test-Parameters: trivial testlist=sanity clientarch=ppc64le env=ONLY=398c,ONLY_REPEAT=100

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: If8c72fa9b13eace009f39daf82454221eba6761b
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56977
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>