security: fix deadlock with prefork

Attempts to start a domain with both SELinux and DAC security
modules loaded will deadlock; latent problem introduced in commit
fdb3bde and exposed in commit 29fe5d7.  Basically, when recursing
into the security manager for other driver's prefork, we have to
undo the asymmetric lock taken at the manager level.

Reported by Jiri Denemark, with diagnosis help from Dan Berrange.

* src/security/security_stack.c (virSecurityStackPreFork): Undo
extra lock grabbed during recursion.

Signed-off-by: Eric Blake <eblake@redhat.com>

src/security/security_stack.c

@@ -127,6 +127,11 @@ virSecurityStackPreFork(virSecurityManagerPtr mgr)
             rc = -1;
             break;
         }
+        /* Undo the unbalanced locking left behind after recursion; if
+         * PostFork ever delegates to driver callbacks, we'd instead
+         * need to recurse to an internal method that does not regrab
+         * a lock. */
+        virSecurityManagerPostFork(item->securityManager);
     }
 
     return rc;