Just acquire a read lock on for preparation.

A read lock will prevent modification of the slots, which is probably
all we really need. Any other thread simply does not survive the fork,
and the current thread can't have a wrtie lock, because we never call
callback or return to the applciation with a write lock held.

Signed-off-by: Simo Sorce <simo@redhat.com>

src/slot.c

@@ -246,7 +246,7 @@ void p11prov_slot_fork_prepare(P11PROV_SLOTS_CTX *sctx)
 {
     int err;
 
-    err = pthread_rwlock_wrlock(&sctx->rwlock);
+    err = pthread_rwlock_rdlock(&sctx->rwlock);
     if (err != 0) {
         err = errno;
         P11PROV_debug("Failed to get slots lock (errno:%d)", err);