LU-1320 llite: fix a race between readpage and releasepage

LU-1320 llite: fix a race between readpage and releasepage

This is a race between page stealing and readpage. If a just read
page is stolen, readpage will find the page is not uptodate, this
makes it panic so -EIO is returned to the reading application.

Signed-off-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Change-Id: Ib16d12d3bc3cc8c0545aa27f0836e4fd89c3a809
Reviewed-on: http://review.whamcloud.com/2591
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: Hudson
Reviewed-by: Bobi Jam <bobijam@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>