migrate_pages: batch flushing TLB

The TLB flushing will cost quite some CPU cycles during the folio
migration in some situations.  For example, when migrate a folio of a
process with multiple active threads that run on multiple CPUs.  After
batching the _unmap and _move in migrate_pages(), the TLB flushing can be
batched easily with the existing TLB flush batching mechanism.  This patch
implements that.

We use the following test case to test the patch.

On a 2-socket Intel server,

- Run pmbench memory accessing benchmark

- Run `migratepages` to migrate pages of pmbench between node 0 and
  node 1 back and forth.

With the patch, the TLB flushing IPI reduces 99.1% during the test and the
number of pages migrated successfully per second increases 291.7%.

Haoxin helped to test the patchset on an ARM64 server with 128 cores, 2
NUMA nodes.  Test results show that the page migration performance
increases up to 78%.

NOTE: TLB flushing is batched only for normal folios, not for THP folios. 
Because the overhead of TLB flushing for THP folios is much lower than
that for normal folios (about 1/512 on x86 platform).

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: "Huang, Ying" <[email protected]>
Tested-by: Xin Hao <[email protected]>
Reviewed-by: Zi Yan <[email protected]>
Reviewed-by: Xin Hao <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: Baolin Wang <[email protected]>
Cc: Oscar Salvador <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Bharata B Rao <[email protected]>
Cc: Alistair Popple <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Hyeonggon Yoo <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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