lustre: Correct KUC code max changelog msg size

lustre: Correct KUC code max changelog msg size

The kernel to userspace communication routines (KUC) allocate
and limit the maximum cs_buf size to CR_MAXSIZE.  However this
fails to account for the fact that the buffer is assumed to begin
with a struct kuc_hdr.  To allocate and account for that space,
we introduce a new define, KUC_CHANGELOG_MSG_MAXSIZE.

Signed-off-by: Christopher J. Morrone <morrone2@llnl.gov>
Reviewed-on: http://review.whamcloud.com/7406
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3587
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: jacques-Charles Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>