EX-8421 llite: remove LBUG() from ll_readpage()

EX-8421 llite: remove LBUG() from ll_readpage()

This LBUG() has been hitting sometimes in sanity-PCC, which
means EX-8421 is not completely fixed.

Until we can fully sort out EX-8421, we don't want to have
this LBUG enabled on customer systems.  The underlying bug
has been present for some time and the first attempt at an
EX-8421 fix improves the situation.

So, remove the LBUG(), with the intent of putting it back
later once EX-8421 is fixed for real.

Fixes: 3a701bf587 ("EX-8421 llite: disable kernel readahead for pcc mmap")
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I21f951d38f67b37626f33068d2a4b64377f4c46a
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52858
Tested-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>