LU-17848 osd: deduplicate OSD locking

LU-17848 osd: deduplicate OSD locking

The two OSDs currently handle locking identically using a rw
semaphore. Future OSDs will likely handle locking in the same
way, considering the need for sleeping; it doesn't make sense to
use internal filesystem locking to handle osd_object lifetimes.
Plus, these locks are only used by upper layers of Lustre - never
internally in the OSD.

Create generic dt semaphore and wrap locking into generic dt_*
functions.

Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: I14df606ee99b94b9b2a610928e8ae5b9b2d8c718
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55148
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Qian Yingjin <qian@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>