Use looser 9pfs caching in VM tests/builds

This can give significant speed ups, see
https://github.com/edolstra/linux/commit/7e20254412c780a2102761fee92cb1d32ceeaefd.

nixos/modules/virtualisation/qemu-vm.nix

···
         ${if cfg.writableStore then "/nix/.ro-store" else "/nix/store"} =
           { device = "store";
             fsType = "9p";
-            options = [ "trans=virtio" "version=9p2000.L" "cache=loose" ];
+            options = [ "trans=virtio" "version=9p2000.L" "veryloose" ];
             neededForBoot = true;
           };
         "/tmp/xchg" =
           { device = "xchg";
             fsType = "9p";
-            options = [ "trans=virtio" "version=9p2000.L" "cache=loose" ];
+            options = [ "trans=virtio" "version=9p2000.L" "veryloose" ];
             neededForBoot = true;
           };
         "/tmp/shared" =

pkgs/build-support/vm/default.nix

···
 
     echo "mounting Nix store..."
     mkdir -p /fs${storeDir}
-    mount -t 9p store /fs${storeDir} -o trans=virtio,version=9p2000.L,cache=loose
+    mount -t 9p store /fs${storeDir} -o trans=virtio,version=9p2000.L,veryloose
 
     mkdir -p /fs/tmp /fs/run /fs/var
     mount -t tmpfs -o "mode=1777" none /fs/tmp
···
 
     echo "mounting host's temporary directory..."
     mkdir -p /fs/tmp/xchg
-    mount -t 9p xchg /fs/tmp/xchg -o trans=virtio,version=9p2000.L,cache=loose
+    mount -t 9p xchg /fs/tmp/xchg -o trans=virtio,version=9p2000.L,veryloose
 
     mkdir -p /fs/proc
     mount -t proc none /fs/proc

pkgs/os-specific/linux/kernel/patches.nix

···
       };
     };
 
+  p9_caching_4_4 = rec
+    { name = "9p-caching.patch";
+      patch = fetchpatch {
+        inherit name;
+        url = https://github.com/edolstra/linux/commit/d522582553368b9564e2d88a8d7b1d469bf98c65.patch;
+        sha256 = "01h7461pdgavd6ghd6w9wg136hkaca0mrmmzhy6s3phksksimbc2";
+      };
+    };
+
 }

pkgs/top-level/all-packages.nix

···
     kernelPatches =
       [ kernelPatches.bridge_stp_helper
         kernelPatches.cpu-cgroup-v2."4.4"
+        kernelPatches.p9_caching_4_4
       ]
       ++ lib.optionals ((platform.kernelArch or null) == "mips")
       [ kernelPatches.mips_fpureg_emu