issue: nbtpage.c,_bt_pagedel may get wrong result

# issue
https://github.com/postgres/postgres/blob/da32f5c4bca7f3447b869de2afbbfa0b74443d45/src/backend/access/nbtree/nbtpage.c#L1943

> /*
> * To avoid deadlocks, we'd better drop the leaf page lock
> * before going further.
> */
> _bt_unlockbuf(rel, leafbuf);
>
> /*
> * Check that the left sibling of leafbuf (if any) is not
> * marked with INCOMPLETE_SPLIT flag before proceeding
> */
> Assert(leafblkno == scanblkno);
> if (_bt_leftsib_splitflag(rel, leftsib, leafblkno))
> {
> ReleaseBuffer(leafbuf);
> return;
> }

After unlocking leaf, but before call _bt_leftsib_splitflag, leftsib may be
already split, and leafbuf's current left sibling is in INCOMPLETE_SPLIT
status.

# how to fix
In function _bt_leftsib_splitflag, after lock leftsib, we should lock
leafbuf again, then recheck if leafbuf's leftsib is still leftsib, if check
passed, keep lock on leafbuf.
--
Regards
Wang Pengfei