LU-5262 osd: only allow set size for regular file

LU-5262 osd: only allow set size for regular file

Otherwise, set size for other non-regular file (such as directory)
will cause the object data corruption.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I44092796961ef26d47ee9c210f52fa451084bd44
Reviewed-on: http://review.whamcloud.com/10870
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>