file-systems: Do not export <file-system>.

* gnu/system/file-systems.scm (<file-system>): Do not export.
* gnu/system.scm (operating-system-root-file-system): Use an accessor
instead of 'match'.

gnu/system.scm

@@ -842,9 +842,8 @@ hardware-related operations as necessary when booting a Linux container."
 
 (define (operating-system-root-file-system os)
   "Return the root file system of OS."
-  (find (match-lambda
-         (($ <file-system> device title "/") #t)
-         (x #f))
+  (find (lambda (fs)
+          (string=? "/" (file-system-mount-point fs)))
         (operating-system-file-systems os)))
 
 (define (operating-system-initrd-file os)

gnu/system/file-systems.scm

@@ -25,8 +25,7 @@
   #:re-export (uuid                               ;backward compatibility
                string->uuid
                uuid->string)
-  #:export (<file-system>
-            file-system
+  #:export (file-system
             file-system?
             file-system-device
             file-system-title