zram: remove entry element member

zram: remove entry element member

Element is in the same anon union as handle and hence holds the same
value, which makes code below sort of confusing

    handle = zram_get_handle()
    if (!handle)
	element = zram_get_element()

Element doesn't really simplify the code, let's just remove it.  We
already re-purpose handle to store the block id a written back page.

Link: https://lkml.kernel.org/r/20241218063513.297475-3-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>