LU-14430 mdd: use own rec_hdr for changelog declare

LU-14430 mdd: use own rec_hdr for changelog declare

Do not use an lu_buf just to declare the changelog record.  This
only needs llog_rec_hdr to pass in lrh_len, so declaring rec_hdr
on the stack avoids the overhead of using the lu_buf.

Fixes: f3d03bc38a ("LU-14430 mdd: fix inheritance of big default ACLs")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I7b6f1d761aa98aa6ecb023894bde03dce23ebbe5
Reviewed-on: https://review.whamcloud.com/43683
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>