usb: gadget: f_mass_storage: remove unnecessary open check

The fsg_lun_is_open() test can be eliminated and the code merged with
the preceding conditional, because the LUN won't be open if
cfg->filename wasn't set. Similarly, the error_lun label will never be
reached with an open lun (non-null filp) so remove the unnecessary
fsg_lun_close() call.

Signed-off-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: Alan Stern <stern@rowland.harvard.edu>
Message-ID: <20230607215401.22563-1-ddiss@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>