r22808: store default buffer size for samr operations in libnet context.

r22808: store default buffer size for samr operations in libnet context.
This allows not requiring it as an argument in some function calls
and still enables specifying any size if it's necessary via libnet
context.

rafal
(This used to be commit 3e4b0c5b3b6d6bd155c8ce11b2342f4af08342f6)