fs/nls: Fix inconsistency between utf8_to_utf32() and utf32_to_utf8()

fs/nls: Fix inconsistency between utf8_to_utf32() and utf32_to_utf8()

After commit 25524b619029 ("fs/nls: Fix utf16 to utf8 conversion"),
the return values of utf8_to_utf32() and utf32_to_utf8() are
inconsistent when encountering an error: utf8_to_utf32() returns -1,
while utf32_to_utf8() returns errno codes. Fix this inconsistency
by modifying utf8_to_utf32() to return errno codes as well.

Fixes: 25524b619029 ("fs/nls: Fix utf16 to utf8 conversion")
Suggested-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Link: https://patch.msgid.link/20251129111535.8984-1-W_Armin@gmx.de
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>