LU-5794 osd: additional checks to verify credits calculation

LU-5794 osd: additional checks to verify credits calculation

every operation counts credits consumed, so we can check when
credits are exceeded. now the number of declarations doesn't
matter for the checks, it's only credits. i.e. it's OK to
declare a single write for a specific range and then make many
writes to that range within a single transaction.

the patch fixes few issues on osd-zfs making it impossible to
declare change to an object being created in a same transaction.

Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Change-Id: I714ccae7ce957e639938df3bed0c4e8fb49d3454
Reviewed-on: http://review.whamcloud.com/12169
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>