master: Define metadata filenames in filesystem.cc

Some time ago there were introduced definitions of filenames
"metadata.mfs.back" and "metadata.mfs.back.tmp" (which aren't all types of
"metadata.mfs" appearing in this source file) but weren't used in every place
in this source file. This commit adds remaining definitions and makes a use of
them.

Change-Id: Ie9d25b4f14464586741a7435ab7eec2d3a603ee1