mkfs-util: Hide /proc/self/mounts before running mkfs

mkfs-util: Hide /proc/self/mounts before running mkfs

mkfs.btrfs refuses to operate on a block device with mounted
partitions, even if doing so is perfectly safe. An example when
this happens is when using systemd-repart with it's --image switch
to add a root partition to a /usr only image. As a workaround until
the issue is fixed, let's hide the information on mounted filesystems
from mkfs.btrfs so it doesn't fail and formats the new filesystem as
expected.