bcache: make is_discard_enabled() static

commit 2d5abb9a upstream.

It's not used outside this file.

Fixes: 63120731 ("bcache: fix failure in journal relplay")
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>