file-systems: Always write the options field in /etc/mtab.

* gnu/build/file-systems.scm (mount-file-system): When OPTIONS is false,
  write "rw" to /etc/mtab.  This fixes a bug whereby 'remove-from-mtab'
  in (guix syscalls) would otherwise raise a 'match-error' because of
  the missing field.  This affected file systems mounted from the
  initrd, such as devtmpfs, which has options == #f.

gnu/build/file-systems.scm

@@ -293,7 +293,7 @@ (define* (mount-file-system spec #:key (root "/root"))
        (mkdir-p (string-append root "/etc"))
        (let ((port (open-file (string-append root "/etc/mtab") "a")))
          (format port "~a ~a ~a ~a 0 0~%"
-                 source mount-point type (or options ""))
+                 source mount-point type (or options "rw"))
          (close-port port))))))
 
 ;;; file-systems.scm ends here