Postpone mutex initialization on FreeBSD.

Postpone mutex initialization on FreeBSD until after base allocation is
safe.
diff --git a/src/jemalloc.c b/src/jemalloc.c
index c701922..a6d2df5 100644
--- a/src/jemalloc.c
+++ b/src/jemalloc.c
@@ -651,6 +651,11 @@
 		return (true);
 	}
 
+	if (mutex_boot()) {
+		malloc_mutex_unlock(&init_lock);
+		return (true);
+	}
+
 	if (opt_narenas == 0) {
 		/*
 		 * For SMP systems, create more than one arena per CPU by