du: avoid to stat all mount points if possible

du: avoid to stat all mount points if possible

du calls stat for each mount point at startup.  This would block or
even make du fail if stat for an unrelated mount point hangs.
The result is not needed in the normal case anyway and therefore
should be avoided.  Issue introduced in commit v8.19-2-gcf7e1b5.

* src/du.c (fill_mount_table): Move function up as it's not used ...
(mount_point_in_fts_cycle): ... here, i.e., the DI_MNT set is
initialized and filled only iff FTS has detected a directory cycle.
(main): Remove the initialization and filling of the DI_MNT set here,
and free the DI_MNT set only if it was used.