LU-1883 osd: Fix niobuf_local offset usage

LU-1883 osd: Fix niobuf_local offset usage

osd-ldiskfs fills file offsets into niobuf_local::offset fields in
osd_map_remote_to_local(), while osd_write_prep() treats the values as
page offsets.  Random memory corruptions may occur as a result of this
offset usage inconsistency.  This patch fixes the issue and introduces
the Orion lnb_file_offset and lnb_page_offset names.

Change-Id: I76c555ddbe7f41271be4bebecbc41ce036747258
Signed-off-by: Li Wei <liwei@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/4051
Tested-by: Hudson
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>