Coccinelle: Add locks/double_lock.cocci

Find double locks.  False positives may occur when some paths cannot
occur at execution, due to the values of variables, and when there is
an intervening function call that releases the lock.

Signed-off-by: Nicolas Palix <npalix@diku.dk>
Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Michal Marek <mmarek@suse.cz>