f2fs: Enable negative dentries on case-insensitive lookup

Instead of invalidating negative dentries during case-insensitive
lookups, mark them as such and let them be added to the dcache.
d_ci_revalidate is able to properly filter them out if necessary based
on the dentry casefold flag.

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
Reviewed-by: Eric Biggers <ebiggers@google.com>
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Changes since v5:
  - Use IS_CASEFOLDED directly (eric)
Changes since v4:
  - Use helper to decide if should set dentry flag.

Changes since v2:
  - Move dentry flag set closer to fscrypt code (Eric)