USB: mark "reject" field of struct urb as atomic_t

It is enough to protect accesses to reject field of urb
by marking it as atomic_t,also it is the only reason of
existence of usb_reject_lock,so remove the lock to make
code more clean.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Acked-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>