fs_mgr: move mkdir(mount_point) to prepare_fs_for_mount()

prepare_fs_for_mount() attempts to mount() the block device at the
mount_point, so it must be created beforehand, as opposed to later in
__mount() where it is done now.

Also fix slight bugs with using fstab.mount_point instead of an
alternative specified mount_point.

Test: boot
Change-Id: I758b6a6efe2dd99433fc1974b69cb6f7ce3bc0c2