[PATCH] parse.c: Make FIO_OPT_STR explicitly one value

The original change had __handle_option() check !first to avoid
parsing an FIO_OPT_STR a second time.  However, the caller handle_option()
has a more explicit check.  Excluding FIO_OPT_STR there is much cleaner.

Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>