[master] b8b798a0f Revert "does the umem backend affect the amount of malloc NULL returns in vtest?"

[Nils Goroll]

commit b8b798a0f59ba99dfe6ba3152ee8ea02182d15cc
Author: Nils Goroll <nils.goroll at uplex.de>
Date:   Tue Jan 14 11:58:41 2020 +0100

    Revert "does the umem backend affect the amount of malloc NULL returns in vtest?"
    
    This reverts commit 8ea006eef403f2126f399d531568a49d1dafa28b.
    
    does not seem to make a difference, trying to narrow down using other
    means (different platforms)

diff --git a/bin/varnishd/storage/storage_umem.c b/bin/varnishd/storage/storage_umem.c
index 644aab5cb..5ce3ce3d7 100644
--- a/bin/varnishd/storage/storage_umem.c
+++ b/bin/varnishd/storage/storage_umem.c
@@ -94,7 +94,7 @@ static umem_cache_destroy_f umem_cache_destroyf = NULL;
 static umem_cache_alloc_f umem_cache_allocf = NULL;
 static umem_cache_free_f umem_cache_freef = NULL;
 
-static const char * const def_umem_options = "perthread_cache=0";
+static const char * const def_umem_options = "perthread_cache=0,backend=mmap";
 static const char * const env_umem_options = "UMEM_OPTIONS";
 
 /* init required per cache get: