fs_mgr: fix double mounting of partition

When looking for partitions to encrypt, it would setup a tmpfs
onto the mount point. But with multi-type mount points it would
repeat the mount.
This would lead to having and extra tmpfs mounted on /data, as
only one would get removed.

Change-Id: I28f2c2c4955906f1172342ea07e0795d6880986b
Signed-off-by: JP Abgrall <jpa@google.com>