LU-5770 osd: find bufsize in declare_xattr_set

LU-5770 osd: find bufsize in declare_xattr_set

mdd_link/mdd_unlink need to know LINKEA buffer size before
transaction start for correct estimation of tx credits.
In case if xattr size not known, make a call to osd_xattr_set()
to find correct buffer size.

Xyratex-bug-id: MRP-2093
Signed-off-by: Alexander Zarochentsev <alexander.zarochentsev@seagate.com>
Change-Id: I890e8182bd2d191a0322d25f0684f2a220873546
Reviewed-on: http://review.whamcloud.com/12412
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: wangdi <di.wang@intel.com>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>