zram: rename init_lock to dev_lock

zram: rename init_lock to dev_lock

init_lock has completely outgrown its initial purpose and is no longer
used only to "prevent concurrent execution of device init" as the stale
comment suggests.  The scope of this lock is much bigger now.

These days this lock (rw_semaphore) controls how a task owns the
corresponding zram device: either in shared mode or in exclusive mode.

All zram device attribute writes should own the device in exclusive mode,
which synchronizes these tasks and prevents, for example, concurrent
execution of recompression and writeback.

All zram device attribute reads should own the device in shared mode.

Rename the lock to dev_lock to better reflect its current purpose.

Link: https://lkml.kernel.org/r/20260115080807.3957860-1-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>