Compare subnet length as well when deleting DHCP entry

When searching for the DHCP subnet entry to delete, the existing code
compares only the subnet prefix and ignores subnet length.  This could
delete the wrong entry/entries if nested subnets are present.

Change-Id: Icf079c42adeca14ef84ec57dc45a5930fde8786d
Closes-Bug: #1362416
Merge "Compare subnet length as well when deleting DHCP entry"