mm: compaction: optimize proactive compaction deferrals

Vlastimil Babka figured out that when fragmentation score didn't go down
across the proactive compaction i.e.  when no progress is made, next wake
up for proactive compaction is deferred for 1 << COMPACT_MAX_DEFER_SHIFT,
i.e.  64 times, with each wakeup interval of
HPAGE_FRAG_CHECK_INTERVAL_MSEC(=500).  In each of this wakeup, it just
decrement 'proactive_defer' counter and goes sleep i.e.  it is getting
woken to just decrement a counter.

The same deferral time can also achieved by simply doing the
HPAGE_FRAG_CHECK_INTERVAL_MSEC << COMPACT_MAX_DEFER_SHIFT thus unnecessary
wakeup of kcompact thread is avoided thus also removes the need of
'proactive_defer' thread counter.

[akpm@linux-foundation.org: tweak comment]

Link: https://lore.kernel.org/linux-fsdevel/88abfdb6-2c13-b5a6-5b46-742d12d1c910@suse.cz/
Link: https://lkml.kernel.org/r/1626869599-25412-1-git-send-email-charante@codeaurora.org
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Khalid Aziz <khalid.aziz@oracle.com>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Nitin Gupta <nigupta@nvidia.com>
Cc: Vinayak Menon <vinmenon@codeaurora.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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