USB: f_fs: the FunctionFS driver

The FunctionFS is a USB composite function that can be used
with the composite framework to create an USB gadget.

>From kernel point of view it is just a composite function with
some unique behaviour.  It may be added to an USB
configuration only after the user space driver has registered
by writing descriptors and strings (the user space program has
to provide the same information that kernel level composite
functions provide when they are added to the configuration).

>From user space point of view it is a file system which when
mounted provide an "ep0" file.  User space driver need to
write descriptors and strings to that file.  It does not need
to worry about endpoints, interfaces or strings numbers but
simply provide descriptors such as if the function was the
only one (endpoints and strings numbers starting from one and
interface numbers starting from core).  The FunctionFS changes
numbers of those as needed also handling situation when
numbers differ in different configurations.

When descriptors and strings are written "ep#" files appear
(one for each declared endpoint) which handle communication on
a single endpoint.  Again, FunctionFS takes care of the real
numbers and changing of the configuration (which means that
"ep1" file may be really mapped to (say) endpoint 3 (and when
configuration changes to (say) endpoint 2)).  "ep0" is used
for receiving events and handling setup requests.

When all files are closed the function disables itself.

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>