mm: compaction: optimize compact_memory to comply with the admin-guide

For the /proc/sys/vm/compact_memory file, the admin-guide states: When 1
is written to the file, all zones are compacted such that free memory is
available in contiguous blocks where possible.  This can be important for
example in the allocation of huge pages although processes will also
directly compact memory as required

But it was not strictly followed, writing any value would cause all zones
to be compacted.

It has been slightly optimized to comply with the admin-guide.  Enforce
the 1 on the unlikely chance that the sysctl handler is ever extended to
do something different.

Commit ef498438 ("mm/compaction: remove unused variable
sysctl_compact_memory") has also been optimized a bit here, as the
declaration in the external header file has been eliminated, and
sysctl_compact_memory also needs to be verified.

[akpm@linux-foundation.org: add __read_mostly, per Mel]
Link: https://lkml.kernel.org/r/tencent_DFF54DB2A60F3333F97D3F6B5441519B050A@qq.com


Signed-off-by: Wen Yang <wenyang.linux@foxmail.com>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: William Lam <william.lam@bytedance.com>
Cc: Pintu Kumar <pintu@codeaurora.org>
Cc: Fu Wei <wefu@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>