Take more care using readlink(2) in device mapping metric
Take more care using readlink(2) in device mapping metric

The newly added hinv.map.lvname metric has an instance domain
which is populated with /dev/mapper links.  The code reading
these links was unaware (as was I) of the quirks of readlink
resulting in occassional failure of pcp/qa test 540.

adfernandes/pcp,andyvand/cygpcpfans,tjanez/pcp,wuliming/pcp,mbaldessari/pcp,andyvand/cygpcpfans,andyvand/cygpcpfans,aeg-aeg/pcpfans,mbaldessari/pcp,edwardt/pcp,prasincs/pcp,aeg-aeg/pcpfans,tjanez/pcp,andyvand/cygpcpfans,tjanez/pcp,aeg-aeg/pcpfans,adfernandes/pcp,andyvand/cygpcpfans,prasincs/pcp,adfernandes/pcp,mbaldessari/pcp,prasincs/pcp,adfernandes/pcp,mbaldessari/pcp,mbaldessari/pcp,edwardt/pcp,aeg-aeg/pcpfans,tjanez/pcp,adfernandes/pcp,wuliming/pcp,aeg-aeg/pcpfans,andyvand/cygpcpfans,tjanez/pcp,edwardt/pcp,prasincs/pcp,mbaldessari/pcp,wuliming/pcp,wuliming/pcp,adfernandes/pcp,adfernandes/pcp,edwardt/pcp,aeg-aeg/pcpfans,edwardt/pcp,aeg-aeg/pcpfans,edwardt/pcp,wuliming/pcp,edwardt/pcp,wuliming/pcp,tjanez/pcp,edwardt/pcp,prasincs/pcp,wuliming/pcp,tjanez/pcp,prasincs/pcp,andyvand/cygpcpfans,wuliming/pcp,aeg-aeg/pcpfans,prasincs/pcp,adfernandes/pcp,prasincs/pcp,andyvand/cygpcpfans,tjanez/pcp