LU-14607 osp: separate buffer for large XATTR

LU-14607 osp: separate buffer for large XATTR

Once XATTR is too large to fit into PAGE_SIZE, allocate value in a
separate buffer for osp_xattr_entry.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Ied090ff73e2e5cdeaf2d91a3670067210f2ab1d7
Reviewed-on: https://review.whamcloud.com/43736
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>