kcopyd use mutex instead of semaphore

Kcopyd uses a semaphore as mutex.  Use the mutex API instead of the (binary)
semaphore,

Signed-off-by: Matthias Kaehlcke <matthias.kaehlcke@gmail.com>
Cc: Neil Brown <neilb@suse.de>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>