LU-18162 lu: cleanup lu_object.h header

LU-18162 lu: cleanup lu_object.h header

Use kernel doc style instead of the old Doxygen style. Avoid
using ** for comments that aren't kernel doc. Fix some
checkpatch.pl issues. Use SPDX.

Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: I9e6e16b49e8ce64053248b182549ef94ab68cf4b
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56117
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>