tipc: Reduce footprint by un-inlining buf_acquire routine

Convert buf_acquire inline routine that is more than one line into
a standard function, thereby eliminating some repeated code.

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>