libdm: ensure first extent is always counted

If FIEMAP returns a single extent after the first call, no extent
boundary is detected and the first extent is not counted by the
normal mechanism.

In this case, increment nr_extents at the same time the extent is
added to the region table, before returning.

libdm/libdm-stats.c

@@ -4286,8 +4286,10 @@ static uint64_t _stats_map_extents(struct dm_pool *mem,
  * If the file only has a single extent, no boundary is ever
  * detected to trigger addition of the first extent.
  */
- if (fm_ext[i - 1].fe_logical == 0)
+ if (fm_ext[i - 1].fe_logical == 0) {
  _stats_add_extent(mem, fm_pending, nr_extents);
+ nr_extents++;
+ }
 
  fiemap->fm_start = (fm_ext[i - 1].fe_logical +
  fm_ext[i - 1].fe_length);