removes the last path component until the mount() succeeds. However,
the code never checks if it has passed the mountpoint, so in some
cases where the mount() never succeeds, it can end up applying the
flags from a mounted filesystem to the underlying one.
Add a sanity check to the code which removes the last path component:
test that the fsid associated with the new path is the same as that
of the old one.
PR: bin/7872
701f37e23c