staging/lustre/obdclass: Proper swabbing of llog_rec_tail.

A variable-length structure preceeds llog_rec_tail within an llog
block. Thus cr_tail shouldn't be accessed directly as a structure
member but its actual location should be computed dynamically.

Signed-off-by: Henri Doreau <henri.doreau@cea.fr>
Reviewed-on: http://review.whamcloud.com/11937
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-5631


Reviewed-by: jacques-Charles Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>