Restore ability to grow primary w/unallocated space before extended (#778700)

A regression which prevented growing a primary partition that had
unallocated space between it and the following extended partition was
introduced with the following commit:

  Create and use general find_extended_partition() function
  aa981077

To fix the regression, restore the logic that checked for a logical
partition before seeking the index of the extended partition.

Bug 778700 - Unable to grow partition even though unallocated space is
             adjacent