fs_mgr: overlayfs: suppress mkfs output

After this change, all the noise from mkfs is suppressed:

$ adb remount
[libfs_mgr]superblock s_max_mnt_count:65535,/dev/block/by-name/system_b
[libfs_mgr]__mount(source=/dev/block/by-name/system_b,target=/mnt/scratch,type=ext4)=-1: Invalid argument
[libfs_mgr]__mount(source=/dev/block/by-name/system_b,target=/mnt/scratch,type=f2fs)=0: Success
Using overlayfs for /system
Using overlayfs for /vendor
Using overlayfs for /product
[libfs_mgr]__mount(source=overlay,target=/system,type=overlay,upperdir=/mnt/scratch/overlay/system/upper)=0
[libfs_mgr]__mount(source=overlay,target=/vendor,type=overlay,upperdir=/mnt/scratch/overlay/vendor/upper)=0
[libfs_mgr]__mount(source=overlay,target=/product,type=overlay,upperdir=/mnt/scratch/overlay/product/upper)=0
remount succeeded

Test: manual as above
Bug: 130739326
Change-Id: I7ed8842e42b74b6a487ce6324b28baf78f1f63bf
diff --git a/fs_mgr/fs_mgr_overlayfs.cpp b/fs_mgr/fs_mgr_overlayfs.cpp
index 8984752..e46e497 100644
--- a/fs_mgr/fs_mgr_overlayfs.cpp
+++ b/fs_mgr/fs_mgr_overlayfs.cpp
@@ -636,7 +636,7 @@
         LERROR << mnt_type << " has no mkfs cookbook";
         return false;
     }
-    command += " " + scratch_device;
+    command += " " + scratch_device + " >/dev/null 2>/dev/null </dev/null";
     fs_mgr_set_blk_ro(scratch_device, false);
     auto ret = system(command.c_str());
     if (ret) {