LU-13473 llite: don't check mirror info for page discard

LU-13473 llite: don't check mirror info for page discard

The CIT_MISC is used for locks/pages manipulation, it will not
go with full io procedure, i.e. cl_io_loop() will not be called
for it. So don't check it for plain file since the mirror info
is not initialized/set in this case.

Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: I723d18260629b8f7c470d350d6d899d3bb88018a
Reviewed-on: https://review.whamcloud.com/38307
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>