ieee1394: dv1394: fix possible deadlock in multithreaded clients

Fix a possible though highly unlikely deadlock:

Thread A:                  Thread B:
 - acquire mmap_sem         - dv1394_ioctl/read/write()
 - dv1394_mmap()            - acquire video->mtx
 - acquire video->mtx       - copy_to/from_user(), possible page fault:
                              acquire mmap_sem

The simplest fix is to use mutex_trylock() instead of mutex_lock() in
dv1394_mmap().  This changes the behavior under contention in a way
which is visible to userspace clients.  However, my guess is that no
clients exist which use mmap vs. ioctl/read/write on the dv1394
character device file interface in concurrent threads.

Reported-by: Johannes Weiner <hannes@saeurebad.de>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>