LU-6489 osd-ldiskfs: check max_page_index correctly

LU-6489 osd-ldiskfs: check max_page_index correctly

This patch fixes a issue that original code check
max_page_index against incorrect variable.

Signed-off-by: Yang Sheng <yang.sheng@intel.com>
Change-Id: I4b164d6187ef4a1c343d4b5803b8915283394037
Reviewed-on: http://review.whamcloud.com/14731
Tested-by: Jenkins
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>