ieee1394: raw1394: replace BKL by local mutex, make ioctl() and mmap() thread-safe

This removes the last usage of the Big Kernel Lock from the ieee1394
stack, i.e. from raw1394's (unlocked_)ioctl and compat_ioctl.

The ioctl()s don't need to take the BKL, but they need to be serialized
per struct file *.  In particular, accesses to ->iso_state need to be
serial.  We simply use a blocking mutex for this purpose because
libraw1394 does not use O_NONBLOCK.  In practice, there is no lock
contention anyway because most if not all libraw1394 clients use a
libraw1394 handle only in a single thread.

mmap() also accesses ->iso_state.  Until now this was unprotected
against concurrent changes by ioctls.  Fix this bug while we are at it.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>