usb: gadget: gadgetfs: Free memory allocated by memdup_user()

Commit 3b74c73f switched over to memdup_user()
in ep_write() function and removed kfree (kbuf).
memdup_user() function allocates memory which is never freed.

Fixes: 3b74c73f (usb: gadget: inode: switch over to memdup_user())
Cc: <stable@vger.kernel.org> # v3.15+
Signed-off-by: Mario Schuknecht <mario.schuknecht@dresearch-fe.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>