USB: g_ffs: the FunctionFS gadget driver

The Function Filesystem (FunctioFS) lets one create USB
composite functions in user space in the same way as GadgetFS
lets one create USB gadgets in user space.  This allows
creation of composite gadgets such that some of the functions
are implemented in kernel space (for instance Ethernet, serial
or mass storage) and other are implemented in user space.

Signed-off-by: Michal Nazarewicz <m.nazarewicz@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>