utils: rm_offset_get_from_fd: Set file_offset based on extent start

Holes are represented by logical gaps in the FIEMAP mapping. Summing the
extent lengths to get a file offset causes file_offset to get out of
sync. If the hole is smaller than the following extent, the next few
FIEMAP calls may return the same extent.

Fixes sahib#529

lib/utilities.c

@@ -1250,7 +1250,7 @@ RmOff rm_offset_get_from_fd(int fd, RmOff file_offset, RmOff *file_offset_next,
             }
 
             /* move offsets in preparation for reading next extent */
-            file_offset += fm_ext.fe_length;
+            file_offset = fm_ext.fe_logical + fm_ext.fe_length;
             expected = fm_ext.fe_physical + fm_ext.fe_length;
         }