migrate_pages: try migrate in batch asynchronously firstly

When we have locked more than one folios, we cannot wait the lock or bit
(e.g., page lock, buffer head lock, writeback bit) synchronously. 
Otherwise deadlock may be triggered.  This make it hard to batch the
synchronous migration directly.

This patch re-enables batching synchronous migration via trying to migrate
in batch asynchronously firstly.  And any folios that are failed to be
migrated asynchronously will be migrated synchronously one by one.

Test shows that this can restore the TLB flushing batching performance for
synchronous migration effectively.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 5dfab109d519 ("migrate_pages: batch _unmap and _move")
Signed-off-by: "Huang, Ying" <[email protected]>
Tested-by: Hugh Dickins <[email protected]>
Reviewed-by: Baolin Wang <[email protected]>
Cc: "Xu, Pengfei" <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Stefan Roesch <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Xin Hao <[email protected]>
Cc: Zi Yan <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Mike Kravetz <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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