namespace: instead of chasing mount symlinks a priori, do so as-we-go

namespace: instead of chasing mount symlinks a priori, do so as-we-go

This is relevant as many of the mounts we try to establish only can be followed
when some other prior mount that is a prefix of it is established. Hence: move
the symlink chasing into the actual mount functions, so that we do it as late
as possibly but as early as necessary.

Fixes: #4588