net: skbuff: improve comment on checksumming

It can be a bit confusing when looking for checksumming flags that
the actual comment for this resides elsewhere further below in the
header file.

Thus, bring the documentation where we define these flags, and
slightly improve the doc text to make it a bit more clear/readable.

Also, whitespace-align values of the define while at it.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>