diskspace: only load filesystem info on demand

diskspace: only load filesystem info on demand

Only load filesystem details for the mount points that we're actually
going to write to. This reduces our syscall count considerably. In the
case of installation, we would actually stat every mountpoint twice (an
extra round for download diskspace) which means (on my system) a total
of 60 syscalls to write to 3 partitions when installing the kernel
package. This change reduces the 60 syscalls down to the expected 3.

A slight debug output change is added here to discern between a
mountpoint added to our linked list versus when we actually load the fs
info.

Signed-off-by: Dave Reisner <dreisner@archlinux.org>
Signed-off-by: Dan McGee <dan@archlinux.org>