Add missing godoc comments for xfrm stats (prometheus#433)

Add missing godoc comments matching the kernel documentation, see
https://www.kernel.org/doc/html/latest/networking/xfrm_proc.html

Also rename the `xfrm*.go` files to have a `net_` prefix, as the
respective file is found in the `net/` sub-directory and this matches
filenames for other network-related stats.

Signed-off-by: Tobias Klauser <[email protected]>

Author: tklauser

xfrm.go renamed to net_xfrm.go

@@ -79,10 +79,13 @@ type XfrmStat struct {
 	// Policy is dead
 	XfrmOutPolDead int
 	// Policy Error
-	XfrmOutPolError     int
-	XfrmFwdHdrError     int
+	XfrmOutPolError int
+	// Forward routing of a packet is not allowed
+	XfrmFwdHdrError int
+	// State is invalid, perhaps expired
 	XfrmOutStateInvalid int
-	XfrmAcquireError    int
+	// State hasn’t been fully acquired before use
+	XfrmAcquireError int
 }
 
 // NewXfrmStat reads the xfrm_stat statistics.