context: when a task fails to acquire an extent lock, don't go ahead and scan the extent anyway

Commit c3b664fea5 ("context: don't forget
to retry locked extents") removed the critical return that prevents a
Task from processing an extent that is locked.

Put the return back.

Fixes: c3b664fea5 ("context: don't forget to retry locked extents")
Signed-off-by: Zygo Blaxell <bees@furryterror.org>

src/bees-context.cc

@@ -721,6 +721,7 @@ BeesContext::scan_forward(const BeesFileRange &bfr_in)
 					// BEESLOGDEBUG("Deferring extent bytenr " << to_hex(extent_bytenr) << " from " << bfr);
 					BEESCOUNT(scanf_deferred_extent);
 					start_over = true;
+					return; // from closure
 				}
 				Timer one_extent_timer;
 				scan_one_extent(bfr, e);