fix #1559: pmxcfs: add missing lock when dumping .rrd

fix #1559: pmxcfs: add missing lock when dumping .rrd

Adding our standard mutex for protecting cfs_status from multiple
conflicting changes solves two things. First, it now protects
cfs_status as it was changed here and secondly, it protects the
global rrd_dump_buf and rrd_dump_last helper variables from
incosistent access and a double free chance.

Fixes: #1559
Reported-by: Tobias Böhm <tb@robhost.de>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>