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
Merged-In: I7ed8842e42b74b6a487ce6324b28baf78f1f63bf