rootless.md: Include GPFS as a parallel filesystem

[jwflory]

Per @rhatdan's request in #3478, this commit makes a note of supporting
General Parallel File System by IBM since it shares the same root issue
as NFS for rootless containers.

[openshift-ci-robot]

Hi @jwflory. Thanks for your PR.

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[rh-atomic-bot]

Can one of the admins verify this patch?
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[mheon]

/approve
/ok-to-test

[openshift-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jwflory, mheon

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[mheon]

LGTM

[rhatdan]

/lgtm
/hold

[giuseppe]

LGTM

/hold cancel