LU-13013 osd: do not count credits for mapped blocks

LU-13013 osd: do not count credits for mapped blocks

this should help to save credits if an application
overwrites using many tiny fragments.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: Ice6541672339f2ba9c4e3d91403a756be323baab
Reviewed-on: https://review.whamcloud.com/36866
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>