qa/cephfs: update _conduct_neg_test_for_root_squash_caps

qa/cephfs: update _conduct_neg_test_for_root_squash_caps

This method was backported in commit
59c9104c54d64e449062d350346b33ad0b3371c9 with modifications instead of
backporting along all commits that were needed. Now that those commmits
have been backported, this method is broken and required modifications
again.

This is a direct commit to Reef branch since it's a direct fix and not a
backport/cherry-pick.

Fixes: https://tracker.ceph.com/issues/65970
Signed-off-by: Rishabh Dave <ridave@redhat.com>