LU-1614 ptlrpc: fix for AT proc read

LU-1614 ptlrpc: fix for AT proc read

This patch fixed a couple of issues in AT proc read:
- calculate of buffer offset is wrong
- current lustre proc read must be less than one page

Signed-off-by: Liang Zhen <liang@whamcloud.com>
Change-Id: Ieef8ad458ca70a4146e0b61fbe8a32b35577ccdf
Reviewed-on: http://review.whamcloud.com/3365
Reviewed-by: Bobi Jam <bobijam@whamcloud.com>
Reviewed-by: wangdi <di.wang@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>