Create a distinct wait event for POSIX DSM allocation.

Previously we displayed "DSMFillZeroWrite" while in posix_fallocate(),
because we shared the same wait event for "mmap" and "posix" DSM types.
Let's introduce a new wait event "DSMAllocate", to be more accurate.

Reported-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20220711174518.yldckniicknsxgzl%40awork3.anarazel.de

doc/src/sgml/monitoring.sgml

@@ -1296,6 +1296,11 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
       <entry><literal>CopyFileWrite</literal></entry>
       <entry>Waiting for a write during a file copy operation.</entry>
      </row>
+     <row>
+      <entry><literal>DSMAllocate</literal></entry>
+      <entry>Waiting for a dynamic shared memory segment to be
+       allocated.</entry>
+     </row>
      <row>
       <entry><literal>DSMFillZeroWrite</literal></entry>
       <entry>Waiting to fill a dynamic shared memory backing file with

src/backend/storage/ipc/dsm_impl.c

@@ -364,7 +364,7 @@ dsm_impl_posix_resize(int fd, off_t size)
 	 */
 	PG_SETMASK(&BlockSig);
 
-	pgstat_report_wait_start(WAIT_EVENT_DSM_FILL_ZERO_WRITE);
+	pgstat_report_wait_start(WAIT_EVENT_DSM_ALLOCATE);
 #if defined(HAVE_POSIX_FALLOCATE) && defined(__linux__)
 	/*
 	 * On Linux, a shm_open fd is backed by a tmpfs file.  If we were to use

src/backend/utils/activity/wait_event.c

@@ -585,6 +585,9 @@ pgstat_get_wait_io(WaitEventIO w)
 		case WAIT_EVENT_DATA_FILE_WRITE:
 			event_name = "DataFileWrite";
 			break;
+		case WAIT_EVENT_DSM_ALLOCATE:
+			event_name = "DSMAllocate";
+			break;
 		case WAIT_EVENT_DSM_FILL_ZERO_WRITE:
 			event_name = "DSMFillZeroWrite";
 			break;

src/include/utils/wait_event.h

@@ -178,6 +178,7 @@ typedef enum
 	WAIT_EVENT_DATA_FILE_SYNC,
 	WAIT_EVENT_DATA_FILE_TRUNCATE,
 	WAIT_EVENT_DATA_FILE_WRITE,
+	WAIT_EVENT_DSM_ALLOCATE,
 	WAIT_EVENT_DSM_FILL_ZERO_WRITE,
 	WAIT_EVENT_LOCK_FILE_ADDTODATADIR_READ,
 	WAIT_EVENT_LOCK_FILE_ADDTODATADIR_SYNC,