LU-16847 ldiskfs: reduce a memory usage by ost IO threads

LU-16847 ldiskfs: reduce a memory usage by ost IO threads

page array is useless once lnb array added it might addressed
via lnb->lnb_page, let's remove it and reduce memory consumption.

Signed-off-by: Alexey Lyashkov <alexey.lyashkov@hpe.com>
Change-Id: Ieb0c186e27f56c770fd2ebfbddce9ccf19791611
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/51392
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>