fs: move the buffer_heads_over_limit stub to buffer_head.h

Move the !CONFIG_BLOCK stub to the same place as the non-stub
declaration.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>