linux-initrd: 'file-system-modules' returns the right module list.

Fixes a bug whereby, for an "iso9660" file system, it would return
'("iso9660" "isofs"), i.e., both the key and the value.

Reported by Danny Milosavljevic <dannym@scratchpost.org>
at <https://debbugs.gnu.org/cgi/bugreport.cgi?bug=30629#115>.

* gnu/system/linux-initrd.scm (lookup-procedure): 'vhash-assoc' returns
a key/value pair; match it.

gnu/system/linux-initrd.scm

@@ -262,8 +262,8 @@ FILE-SYSTEMS."
      (let ((table (vhash mapping ...)))
        (lambda (key)
          (match (vhash-assoc key table)
-           (#f    default)
-           (value value)))))))
+           (#f            default)
+           ((key . value) value)))))))
 
 (define file-system-type-modules
   ;; Given a file system type, return the list of modules it needs.