libmount: cleanup --fake mode

libmount: cleanup --fake mode

It was originally designed to play nasty games with /etc/mtab (mount
when /etc is read-only and update later when it's write-able).

The --fake is completely useless with the new API due to complexity
where we cannot skip any step, because the next stuff depends on it.
So, it makes more sense skip all functionality where libmount does
anything significant.

This commit add --fake check to hooks logic to skip all hooks as the
hooks are place where libmount implements mount related invasive
operations (create mountpoint, namespaces, create superblock, move,
mount, etc.).

Frankly, --fake without mtab is useless.

Fixes: https://github.com/util-linux/util-linux/issues/2395
Signed-off-by: Karel Zak <kzak@redhat.com>