makefs: Handle multiple staging directories when creating ZFS pools

The fsnode tree traversal routines used in ZFS mode assume that all
children of a (directory) fsnode can be accessed using a directory fd
for the parent and the child name. This is true when populating the
image using an mtree manifest or from a single staging directory, but
doesn't work when multiple staging directories are specified.

Change the traversal routines to use absolute path lookups when an mtree
manifest is not in use. This isn't ideal, but it's the simplest way to
fix the problem.

Reported by: imp
Sponsored by: The FreeBSD Foundation