zram: rename __zram_bvec_read to zram_read_page

__zram_bvec_read doesn't get passed a bvec, but always read a whole page. 
Rename it to make the usage more clear.

Link: https://lkml.kernel.org/r/20230411171459.567614-10-hch@lst.de


Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>