jffs2: fix error value sign

do_verify_xattr_datum(), do_load_xattr_datum(), load_xattr_datum()
and verify_xattr_ref() should return negative value on error.
Sometimes they return EIO that is positive.  Change this to -EIO.

Signed-off-by: Vasiliy Kulikov <segoon@openwall.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>