LU-8837 lustre: remove target declarations from obd.h

LU-8837 lustre: remove target declarations from obd.h

lu_target.h and obd_target.h are only needed in obd.h
for some structs in obd_device.u.  We don't really need to mention
these structs in the union as they are all quite small.

So we can define accessor function that cast a pointer to the union
into the required type, and then we can completely remove these
includes from obd.h

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I9b314b0bfc1baae03ccb8eadf134964ea308f638
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/41952
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>