ovl: support layers on case-folding capable filesystems

Case folding is often applied to subtrees and not on an entire
filesystem.

Disallowing layers from filesystems that support case folding is over
limiting.

Replace the rule that case-folding capable are not allowed as layers
with a rule that case folded directories are not allowed in a merged
directory stack.

Should case folding be enabled on an underlying directory while
overlayfs is mounted the outcome is generally undefined.

Specifically in ovl_lookup(), we check the base underlying directory
and fail with -ESTALE and write a warning to kmsg if an underlying
directory case folding is enabled.

Suggested-by: Kent Overstreet <[email protected]>
Link: https://lore.kernel.org/linux-fsdevel/[email protected]/
Signed-off-by: Amir Goldstein <[email protected]>
Link: https://lore.kernel.org/[email protected]
Reviewed-by: Kent Overstreet <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

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