Fix GIN VACUUM posting tree root split bug.

ginVacuumPostingTreeLeaves swaps a shared buffer lock for an exclusive
one when it encounters a leaf page.  It neglected to re-verify whether a
page that was initially a leaf root page became an internal page due to
a concurrent root page split (during the window when no lock was held).
It was therefore possible for GIN VACUUM to spuriously treat an internal
page as a leaf page, leading to data corruption.  VACUUM could miss dead
TIDs that it was required to remove, leaving behind dangling references
in the index.

To fix, re-verify that a leaf page is still a leaf page after an
exclusive lock is acquired.  If it isn't, drop our exclusive lock and
acquire a shared lock so that the non-leaf root page gets processed in
the usual way.

Oversight in commit fd83c83d, which fixed a deadlock bug in GIN posting
tree vacuuming.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-by: Andrey Borodin <x4mmm@yandex-team.ru>
Discussion: https://postgr.es/m/CAH2-Wz=RBpJTQgvOxr6C=J04dExmFSt1E3F-r+cRTQ56hEotkg@mail.gmail.com
Backpatch-through: 14

src/backend/access/gin/ginvacuum.c

@@ -366,6 +366,16 @@ ginVacuumPostingTreeLeaves(GinVacuumState *gvs, BlockNumber blkno)
 		{
 			LockBuffer(buffer, GIN_UNLOCK);
 			LockBuffer(buffer, GIN_EXCLUSIVE);
+
+			if (!GinPageIsLeaf(page))
+			{
+				/*
+				 * The root page was a leaf page, but became an internal page
+				 * while no lock was held.  Unlock and reacquire a share lock.
+				 */
+				UnlockReleaseBuffer(buffer);
+				continue;
+			}
 			break;
 		}