rtnetlink-link: Make 'attrs' local variable non-static.

I don't see any reason for this variable to be static.

CC: Ethan Jackson <ethan@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
ofp-print: Avoid returning static data.

Returning a static data buffer makes code more brittle and definitely
not thread-safe, so this commit switches to using a caller-provided
buffer instead.

Signed-off-by: Ben Pfaff <blp@nicira.com>