EX-7596 tests: don't fail if metadata_csum_seed unset

EX-7596 tests: don't fail if metadata_csum_seed unset

Fix logic in the sanity-pcc cache filesystem setup.  With mke2fs
1.47.0-wc1 it enabled metadata_csum_seed unconditionally, but it
caused problems on el7.9 kernels. In 1.47.0-wc2 it disabled that
feature, caused the check for removing the feature to fail.

Since metadata_csum_seed has been available since 1.44.2 it
shouldn't be a problem to force it off duing mke2fs.

Test-Parameters: trivial testlist=sanity-pcc env=ONLY=1
Fixes: 7d35dd13273e7 ("EX-7596 tests: disable metadata_csum_seed for pcc cache device")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ic04ab4043981dc9b5c32e01c4aa85be343e3f3f8
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/51417
Reviewed-by: Feng Lei <flei@whamcloud.com>
Reviewed-by: Li Dongyang <dongyangli@ddn.com>
Tested-by: jenkins <devops@whamcloud.com>