Fix undefined reference to `libzfs_fru_compare'

Add trivial libzfs_fru_compare() function which can be used when
HAVE_LIBTOPO is not defined.  The only caller is find_vdev() and
this function should never be reached because search_fru must be
NULL unless HAVE_LIBTOPO is defined.

Rename _HAS_FMD_TOPO to existing HAVE_LIBTOPO which was
originally added for this purpose.  This macro will never be defined.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #5402
Closes #5909

lib/libzfs/libzfs_fru.c

@@ -455,6 +455,15 @@ libzfs_fru_clear(libzfs_handle_t *hdl, boolean_t final)
 
 #else /* HAVE_LIBTOPO */
 
+/*
+ * Compare to two FRUs, ignoring any authority information.
+ */
+boolean_t
+libzfs_fru_compare(libzfs_handle_t *hdl, const char *a, const char *b)
+{
+	return (B_FALSE);
+}
+
 /*
  * Clear memory associated with the FRU hash.
  */