udf: Explain handling of load_nls() failure

[ Upstream commit a768a9abc625d554f7b6428517089c193fcb5962 ]

Add comment explaining that load_nls() failure gets handled back in
udf_fill_super() to avoid false impression that it is unhandled.

Signed-off-by: Jan Kara <jack@suse.cz>
Stable-dep-of: fc8033a34a3c ("udf: Preserve link count of system files")
Signed-off-by: Sasha Levin <sashal@kernel.org>