posix: checkDiskFree() also checks free inodes. (#2086)

Previously checkDiskFree() checks for free available space.  This
patch enables checkDiskFree() also checks for free inodes in linux and
free clusters in windows.

Fixes #2075

pkg/disk/disk.go

@@ -23,5 +23,7 @@ package disk
 type Info struct {
 	Total  int64
 	Free   int64
+	Files  int64
+	Ffree  int64
 	FSType string
 }