LU-15067 lod: fix error handling in lod_new_pool

LU-15067 lod: fix error handling in lod_new_pool

- correct error handling in lod_new_pool - ENOMEM
 from tgt_pool_init may cause incorrect pool_count.
- optimisation in lu_tgt_pool_add. Do not extend
 a pool, if the target is already exists.

HPE-bug-id: LUS-6995
Change-Id: I0ea00d335217f3994334bd02ae36c34653e7da98
Reviewed-by: Alexander Zarochentsev <c17826@cray.com>
Reviewed-by: Artem Blagodarenko <artem.blagodarenko@hpe.com>
Signed-off-by: Sergey Cheremencev <sergey.cheremencev@hpe.com>
Reviewed-on: https://review.whamcloud.com/45137
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>