usb: f_mass_storage: limit number of reported LUNs

usb: f_mass_storage: limit number of reported LUNs

commit 8515bac01a983d277148e4fcc5f235bf603de577 upstream.

Mass storage function created via configfs always reports eight LUNs
to the hosts even if only one LUN has been configured.  Adjust the
number when the USB function is allocated based on LUNs that user
has created.

Tested-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>