mm: thp: don't need to drain lru cache when splitting and mlocking THP

mm: thp: don't need to drain lru cache when splitting and mlocking THP

Since commit 4f1d33f5ce94 ("mm/swap.c: flush lru pvecs on compound page
arrival") THP would not stay in pagevec anymore.  So the optimization made
by commit 3166d3d92f77 ("thp: increase split_huge_page() success rate")
doesn't make sense anymore, which tries to unpin munlocked THPs from
pagevec by draining pagevec.

Draining lru cache before isolating THP in mlock path is also unnecessary.
634720d40076 ("mm, thp: fix mapped pages avoiding unevictable list on
mlock") added it and 28d7bdb0830c ("thp, mlock: do not mlock PTE-mapped
file huge pages") accidentally carried it over after the above
optimization went in.

Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Link: http://lkml.kernel.org/r/1585946493-7531-1-git-send-email-yang.shi@linux.alibaba.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>