btrfs: unexport btrfs_run_discard_work and make it static

Mark btrfs_run_discard_work static and move it above its callers.

Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>