mountpoint: don't hardcode paths

Signed-off-by: Karel Zak <kzak@redhat.com>
lslocks: use libmount to parse mountinfo

old version:
	$ strace -e open lslocks 2>&1 | grep mountinfo | wc -l
	118

new version:
	strace -e open ./lslocks 2>&1 | grep mountinfo | wc -l
	1

Reported-by: Bernhard Voelker <mail@bernhard-voelker.de>
Signed-off-by: Karel Zak <kzak@redhat.com>