zram: pass estimated src size hint to zstd

zram works with PAGE_SIZE buffers, so we always know exact size of the
source buffer and hence can pass estimated_src_size to zstd_get_params().

This hint on x86_64, for example, reduces the size of the work memory
buffer from 1303520 bytes down to 90080 bytes.  Given that compression
streams are per-CPU that's quite some memory saving.

Link: https://lkml.kernel.org/r/20240902105656.1383858-10-senozhatsky@chromium.org


Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nick Terrell <terrelln@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>