udf: Allow mounting volumes with incorrect identification strings

Commit c26f6c61 ("udf: Fix conversion of 'dstring' fields to UTF8")
started to be more strict when checking whether converted strings are
properly formatted. Sudip reports that there are DVDs where the volume
identification string is actually too long - UDF reports:

[  632.309320] UDF-fs: incorrect dstring lengths (32/32)

during mount and fails the mount. This is mostly harmless failure as we
don't need volume identification (and even less volume set
identification) for anything. So just truncate the volume identification
string if it is too long and replace it with 'Invalid' if we just cannot
convert it for other reasons. This keeps slightly incorrect media still
mountable.

CC: stable@vger.kernel.org
Fixes: c26f6c61 ("udf: Fix conversion of 'dstring' fields to UTF8")
Reported-and-tested-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>