block: mark __bio_try_merge_page static

Mark __bio_try_merge_page static and move it up a bit to avoid the need
for a forward declaration.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211012161804.991559-7-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>