LU-3680 osd: reduce osd_thread_info in ldiskfs osd

LU-3680 osd: reduce osd_thread_info in ldiskfs osd

by unioning few rarely used fields. now the structure should
fit a page:

(gdb) p sizeof(struct osd_thread_info)
$1 = 3296

Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Change-Id: I75d5c6fefa41884390ce155781e0963884a3ad2c
Reviewed-on: http://review.whamcloud.com/9726
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>