isofs: joliet: Fix iocharset=utf8 mount option

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_iocharset is set to NULL. So
simplify code around, remove s_utf8 field as to distinguish between UTF-8
and non-UTF-8 it is needed just to check if s_nls_iocharset is set to NULL
or not.

Link: https://lore.kernel.org/r/20210808162453.1653-5-pali@kernel.org


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