LU-5631 obdclass: Proper swabbing of llog_rec_tail.

LU-5631 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>
Change-Id: I2d244797d107cf52f647e19b2db780138e910925
Reviewed-on: http://review.whamcloud.com/11953
Tested-by: Jenkins
Reviewed-by: jacques-Charles Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>