LU-13431 llite: rework preferred IO size

LU-13431 llite: rework preferred IO size

Instead of using inode->i_blkbits to infer preferred IO size,
just set stat->blksize to the right value in ll_getattr_dentry().

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: If705c7d52bdfabdd3e669ca2d34f8cc0ca1ae08a
Reviewed-on: https://review.whamcloud.com/38176
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>