libdm: break up _stats_get_extents_for_file()

libdm: break up _stats_get_extents_for_file()

Split out the loop that iterates over each batch of FIEMAP
extent data from the function that sets up and calls the ioctl
to reduce nesting and simplify local variable use:

  _stats_get_extents_for_file()
  ->  _stats_map_extents()

The _stats_map_extents() function is responsible for detecting
eof and extent boundaries and adding whole, allocated extents
to the file extent table for region creation.