case-insensitive updates for 6.9

- Patch case-insensitive lookup by trying the case-exact comparison
first, before falling back to costly utf8 casefolded comparison.

- Fix to forbid using a case-insensitive directory as part of an
overlayfs mount.

- Patchset to ensure d_op are set at d_alloc time for fscrypt and
casefold volumes, ensuring filesystem dentries will all have the correct
ops, whether they come from a lookup or not.
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