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

Since commit 8f182270 ("mm/swap.c: flush lru pvecs on compound page
arrival") THP would not stay in pagevec anymore.  So the optimization made
by commit d9654322 ("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.
b676b293 ("mm, thp: fix mapped pages avoiding unevictable list on
mlock") added it and 9a73f61b ("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>