zram: rework bdev block allocation

zram: rework bdev block allocation

First, writeback bdev ->bitmap bits are set only from one context, as we
can have only one single task performing writeback, so we cannot race with
anything else.  Remove retry path.

Second, we always check ZRAM_WB flag to distinguish writtenback slots, so
we should not confuse 0 bdev block index and 0 handle.  We can use first
bdev block (0 bit) for writeback as well.

While at it, give functions slightly more accurate names, as we don't
alloc/free anything there, we reserve a block for async writeback or
release the block.

Link: https://lkml.kernel.org/r/20251122074029.3948921-6-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Brian Geffon <bgeffon@google.com>
Cc: Minchan Kim <minchan@google.com>
Cc: Richard Chang <richardycc@google.com>
Cc: Yuwen Chen <ywen.chen@foxmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>