ext4: Simplify the handling of cached casefolded names

ext4: Simplify the handling of cached casefolded names

Keeping it as qstr avoids the unnecessary conversion in ext4_match

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
[eugen.hristev@collabora.com: port to 6.10-rc1]
Signed-off-by: Eugen Hristev <eugen.hristev@collabora.com>
Link: https://lore.kernel.org/r/20240606073353.47130-2-eugen.hristev@collabora.com
Reviewed-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Gabriel Krisman Bertazi <krisman@suse.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>