Use very long timeout for information in the filehandle->path cache.

	/* The fsid -> path lookup can be quite expensive as it
	 * potentially stats and reads lots of devices, and some of those
	 * might have spun-down.  The Answer is not likely to
	 * change underneath us, and an 'exportfs -f' can always
	 * remove this from the kernel, so use a really log
	 * timeout.  Maybe this should be configurable on the command
	 * line.
	 */

Signed-off-by: Neil Brown <neilb@suse.de>