Assert unlock doesn't underflow lock user count.

While debugging a double unlock error we hit this condition and
debugging would have been a lot easier had we enforced this simple
constraint that we can't decrement the lock users count if it's
already 0.

Signed-off-by: Auke Kok <auke.kok@versity.com>

kmod/src/lock.c

@@ -302,6 +302,7 @@ static void lock_inc_count(unsigned int *counts, enum scoutfs_lock_mode mode)
 static void lock_dec_count(unsigned int *counts, enum scoutfs_lock_mode mode)
 {
 	BUG_ON(mode < 0 || mode >= SCOUTFS_LOCK_NR_MODES);
+	BUG_ON(counts[mode] == 0);
 	counts[mode]--;
 }