namespace: store and use original MountEntry paths when prefixing

namespace: store and use original MountEntry paths when prefixing

Some paths (eg: mount_tmpfs) simply assumed that prefixing always
happens and it always stores the original path in path_const, and
the prefixed path in path_malloc.
But if a MountEntry is set up in a helper function and thus uses
only _malloc struct members, this assumption doesn't hold and there's
a crash.

Refactor so that prefixing is done with a helper which stores the
original path in a separate struct member, and accessing it also
uses a helper which does the right thing.