USB: snoop processes opening usbfs device files

This patch (as1148) adds a new "snoop" message to usbfs when a device
file is opened, identifying the process responsible.  This comes in
extremely handy when trying to determine which program is doing some
unwanted USB access.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>