Fix race in dsm_unpin_segment() when handles are reused.

Teach dsm_unpin_segment() to skip segments that are in the process
of being destroyed by another backend, when searching for a handle.
Such a segment cannot possibly be the one we are looking for, even
if its handle matches.  Another slot might hold a recently created
segment that has the same handle value by coincidence, and we need
to keep searching for that one.

The bug caused rare "cannot unpin a segment that is not pinned"
errors on 10 and 11.  Similar to commit 6c0fb941 for dsm_attach().

Back-patch to 10, where dsm_unpin_segment() landed.

Author: Thomas Munro
Reported-by: Justin Pryzby
Tested-by: Justin Pryzby (along with other recent DSA/DSM fixes)
Discussion: https://postgr.es/m/20190216023854.GF30291@telsasoft.com

src/backend/storage/ipc/dsm.c

@@ -891,8 +891,8 @@ dsm_unpin_segment(dsm_handle handle)
 	LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE);
 	for (i = 0; i < dsm_control->nitems; ++i)
 	{
-		/* Skip unused slots. */
-		if (dsm_control->item[i].refcnt == 0)
+		/* Skip unused slots and segments that are concurrently going away. */
+		if (dsm_control->item[i].refcnt <= 1)
 			continue;
 
 		/* If we've found our handle, we can stop searching. */