udf: Simplify calls to udf_disk_stamp_to_time

Subsequent patches in the series convert inode timestamps
to use struct timespec64 instead of struct timespec as
part of solving the y2038 problem.

commit fd3cfad3 ("udf: Convert udf_disk_stamp_to_time() to use mktime64()")
eliminated the NULL return condition from udf_disk_stamp_to_time().
udf_time_to_disk_time() is always called with a valid dest pointer and
the return value is ignored.
Further, caller can as well check the dest pointer being passed in rather
than return argument.
Make both the functions return void.

This will make the inode timestamp conversion simpler.

Signed-off-by: Deepa Dinamani <deepa.kernel@gmail.com>
Cc: jack@suse.com

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Changes from v1:
* fixed the pointer error pointed by Jan