`IsLikelyNotMountPoint()` is an optimized version for `IsMountPoint()`
which can not detect all type of mounts (anymore). The slower
`IsMountPoint()` is more safe to use. This can cause a slight
performance regression in the case there are many mountpoints on the
system, but correctness is more important than speed while mounting.
Fixes: #4633
Signed-off-by: Niels de Vos <ndevos@ibm.com>