LU-16796 lov: Change struct pool_desc to use kref

LU-16796 lov: Change struct pool_desc to use kref

This patch changes struct pool_desc to use
kref(refcount_t) instead of atomic_t

Signed-off-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Change-Id: I3829de190ec148c2e087f6a0262bf3bb76c196af
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52122
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>