LU-1051 osd: reserve less credits for llog unlink

LU-1051 osd: reserve less credits for llog unlink

Reserve less credits for the llog write and catalog header udpate.
Since we can't get the llog inode in osd declare function, we use
a temporary solution: hack the 'size' parameter to indicate a llog
write or catalog header udpate.

Signed-off-by: Niu Yawei <niu@whamcloud.com>
Change-Id: Ic5df00a878cdbf64a44b22b684e2c6b2dff83b07
Reviewed-on: http://review.whamcloud.com/2100
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>