usb: gadget: f_fs: check for allocation failure

Return -ENOMEM if kmalloc() fails.

Fixes: 9353afbb ('usb: gadget: f_fs: buffer data from ‘oversized’ OUT requests')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>