netgraph/ng_base: Allow larger BINARY2ASCII conversions

Allocate the necessary memory for the conversion dynamically starting
with a value which is sufficient for almost all normal cases.

PR: 187835
Reviewed by: kp
Differential Revision: https://reviews.freebsd.org/D23840

(cherry picked from commit 45d75e3ac3fb5bf8230ca28dc09b48c6e5ed7a4f)