udf: Avoid unneeded variable length array in struct fileIdentDesc

impUse variable length array in struct fileIdentDesc is never used.
It serves only for documentation purposes of the on-disk format. Remove
it from the struct so that it doesn't confuse the compiler and
reviewers.

Reported-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
References: https://lore.kernel.org/all/ZRrsYkKIQe8K6F/t@work


Signed-off-by: Jan Kara <jack@suse.cz>