misc: refactor calc_checksums

misc: refactor calc_checksums

Merge two mildly incompatible static inlines, and rename the results
vnet_calc_checksums_inline (...).

The resulting inline has three additional parameters: int is_ip4, int
is_ip6, and int with_gso. All calls manage to pass one or more as
compile-time constants, which causes a certain amount of code to
disappear in each instantiation.

Type: refactor
Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: I1a2a0e353b9a7bca20bc81318e8c915617261e1a