LU-11288 osc: re-check target versus available grant

LU-11288 osc: re-check target versus available grant

- under the spinlock, otherwise it's possible that available
  grant has changed since target calculation and bytes to
  shrink go negative.
- tgt_grant_alloc() should avoid negative grants

Change-Id: I35613e4e840e172977c7b866fb429c40a7fefc8f
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/33226
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>