Strip any trailing whitespace from rbd showmapped

More recent versions of ceph append a bit of whitespace to the line
after the name of the /dev/rbdX device; this causes the monitor check
to fail as it can't find the device name due to the whitespace.

This fix excludes any characters after the /dev/rbdN match.
(cherry picked from commit ad84ea07cac5096de38b51b8fc452c99f016b8d8)