zram: cond_resched() in writeback loop

zram writeback is a costly operation, because every target slot (unless
ZRAM_HUGE) is decompressed before it gets written to a backing device. 
The writeback to a backing device uses submit_bio_wait() which may look
like a rescheduling point.  However, if the backing device has
BD_HAS_SUBMIT_BIO bit set __submit_bio() calls directly
disk->fops->submit_bio(bio) on the backing device and so when
submit_bio_wait() calls blk_wait_io() the I/O is already done.  On such
systems we effective end up in a loop

    for_each (target slot) {
	decompress(slot)
	__submit_bio()
	    disk->fops->submit_bio(bio)
    }

Which on PREEMPT_NONE systems triggers watchdogs (since there are no
explicit rescheduling points).  Add cond_resched() to the zram writeback
loop.

Link: https://lkml.kernel.org/r/20241218063513.297475-8-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>

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