udf: Fix iocharset=utf8 mount option

[ Upstream commit b645333443712d2613e4e863f81090d5dc509657 ]

Currently iocharset=utf8 mount option is broken. To use UTF-8 as iocharset,
it is required to use utf8 mount option.

Fix iocharset=utf8 mount option to use be equivalent to the utf8 mount
option.

If UTF-8 as iocharset is used then s_nls_map is set to NULL. So simplify
code around, remove UDF_FLAG_NLS_MAP and UDF_FLAG_UTF8 flags as to
distinguish between UTF-8 and non-UTF-8 it is needed just to check if
s_nls_map set to NULL or not.

Link: https://lore.kernel.org/r/20210808162453.1653-4-pali@kernel.org


Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Sasha Levin <sashal@kernel.org>