Use LW_SHARED in dsa.c where possible.

Both dsa_get_total_size() and dsa_get_total_size_from_handle() take
an exclusive lock just to read a variable.  This commit reduces the
lock level to LW_SHARED in those functions.

Reviewed-by: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/aS8fMzWs9e8iHxk2%40nathan

src/backend/utils/mmgr/dsa.c

@@ -1043,7 +1043,7 @@ dsa_get_total_size(dsa_area *area)
 {
 	size_t		size;
 
-	LWLockAcquire(DSA_AREA_LOCK(area), LW_EXCLUSIVE);
+	LWLockAcquire(DSA_AREA_LOCK(area), LW_SHARED);
 	size = area->control->total_segment_size;
 	LWLockRelease(DSA_AREA_LOCK(area));
 
@@ -1075,7 +1075,7 @@ dsa_get_total_size_from_handle(dsa_handle handle)
 
 	control = (dsa_area_control *) dsm_segment_address(segment);
 
-	LWLockAcquire(&control->lock, LW_EXCLUSIVE);
+	LWLockAcquire(&control->lock, LW_SHARED);
 	size = control->total_segment_size;
 	LWLockRelease(&control->lock);