Destroy the pmap 'pm_mutex' in pmap_release() otherwise we will panic

subsequently in pmap_pinit() with the following signature:

panic: lock "pmap" 0xc7878bc8 already initialized

This bug was uncovered by the changes made to vm_map.c in r206140.

sys/mips/mips/pmap.c

@@ -1263,6 +1263,7 @@ pmap_release(pmap_t pmap)
 	ptdpg->wire_count--;
 	atomic_subtract_int(&cnt.v_wire_count, 1);
 	vm_page_free_zero(ptdpg);
+	PMAP_LOCK_DESTROY(pmap);
 }
 
 /*