libmount: optimize mtab and utab parsing in umount

libmount: optimize mtab and utab parsing in umount

create 8000 NFS mountpoints:
	#!/bin/bash
	mount=/tmp/mount
	if [ ! -d $mount ]; then
	    mkdir -p $mount
	fi
	for dir in {1..8000}; do
	    if [ ! -d $mount/$dir ]; then
		mkdir -p $mount/$dir
	    fi
	    echo mount $dir
	    mount -t nfs 127.0.0.1:/ $mount/$dir
	done

old version:
	time ./umount /tmp/mount/2255

	real	0m1.254s
	user	0m1.002s
	sys	0m0.238s

new version:
	time ./umount /tmp/mount/2244

	real	0m0.332s
	user	0m0.111s
	sys	0m0.218s

Reported-by: chenditang <chendt.fnst@cn.fujitsu.com>
Signed-off-by: Karel Zak <kzak@redhat.com>