tipc: Eliminate trivial buffer manipulation helper routines

Gets rid of two inlined routines that simply call existing sk_buff
manipulation routines, since there is no longer any extra processing
done by the helper routines.

Note that these changes are essentially cosmetic in nature, and have
no impact on the actual operation of TIPC.

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>