zram: pass buffer offset to zcomp_available_show()

zram: pass buffer offset to zcomp_available_show()

In most cases zcomp_available_show() is the only emitting
function that is called from sysfs read() handler, so it
assumes that there is a whole PAGE_SIZE buffer to work with.
There is an exception, however: recomp_algorithm_show().

In recomp_algorithm_show() we prepend the buffer with
priority number before we pass it to zcomp_available_show(),
so it cannot assume PAGE_SIZE anymore and must take
recomp_algorithm_show() modifications into consideration.
Therefore we need to pass buffer offset to zcomp_available_show().

Also convert it to use sysfs_emit_at(), to stay aligned
with the rest of zram's sysfs read() handlers.

On practice we are never even close to using the whole PAGE_SIZE
buffer, so that's not a critical bug, but still.

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Link: https://lore.kernel.org/r/20250627071840.1394242-1-senozhatsky@chromium.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>