LU-9679 mdt: use OBD_ALLOC_PTR_ARRAY() and FREE

LU-9679 mdt: use OBD_ALLOC_PTR_ARRAY() and FREE

Use:
  OBD_ALLOC_PTR_ARRAY
  OBD_FREE_PTR_ARRAY

for allocating and freeing arrays in mdt.

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I5e69f03370fc42fdfe770a4c6fc72a994a6749f6
Reviewed-on: https://review.whamcloud.com/38251
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>