fs: dlm: drop rxbuf manipulation in dlm_copy_master_names

This patch removes the manipulation of the receive buffer in case of an
error and be sure the buffer is null terminated before an error
messagea is printed out. Instead of manipulate the receive buffer we
tell inside the format string the maximum length the string buffer is
being read.

Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>