Coccinelle: Add locks/mini_lock.cocci

Find missing unlocks.  This semantic match considers the specific case
where the unlock is missing from an if branch, and there is a lock
before the if and an unlock after the if.  False positives are due to
cases where the if branch represents a case where the function is
supposed to exit with the lock held, or where there is some preceding
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>