libmount: don't call hooks after mount.<type> helper

libmount: don't call hooks after mount.<type> helper

In case more filesystems are specified (or when libmount follows
/{etc,proc}/filesystems) then the library may try to use and
initialize the new API because for some filesystems, we need
exec(/sbin/mount.<type>) and for another fsopen().

The hooks that use the API have to smart and detect that the mount
operation was done in external /sbin/mount.<type> helper. And in this
case, the new API file descriptors must be ignored.

The exception is propagation flags, mount(8) can set the flags after
exec(/sbin/mount.<type>), for example, "mount -t ntfs --make-private".

Fixes: https://github.com/util-linux/util-linux/issues/2267
Signed-off-by: Karel Zak <kzak@redhat.com>