LU-7371 osd-ldiskfs: fix wrong read length over isize

LU-7371 osd-ldiskfs: fix wrong read length over isize

If the isize is 4095, a read length of 4096 will be
returned because a wrong calculation of EOF. This patch fixes the
problem.

Signed-off-by: Li Xi <lixi@ddn.com>
Change-Id: I73b18641f000a2d96067243c08c26e51d0d53244
Reviewed-on: http://review.whamcloud.com/17020
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>