usbmon: restore mmap

Paolo asked to enable the mmap. I kept it off because I'm do not
entirely understand how it workse these days after ->nopage etc.
But it seems like working somewhat at least.

Signed-Off-By: Pete Zaitcev <zaitcev@gmail.com>
Cc: Paolo Abeni <paolo.abeni@email.it>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>