LU-17469 llite: hold object reference in IO

LU-17469 llite: hold object reference in IO

There could be a race between page write and inode free, hold
a cl_object reference during the IO lest accessing freed object.

Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: Ic70cc27430e68265aba0662fc68e9bfe2f86cfe1
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/53819
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>