net: rmnet_data: Add support for data path testing in UML

When configuring networking using mcast and ethernet, we need to
stamp a dummy 14 bytes header to each MAP packet. Add support
for this by using the RMNET_EGRESS_FORMAT__RESERVED__ egress data
format option.

CRs-Fixed: 2086060
Change-Id: Ie586fa4381badcd4649fa736557b93dbbc5c102d
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>