libnetlink: flush stdout only in monitor mode

libnetlink: flush stdout only in monitor mode

As Eric pointed out, fflush() only makes sense in monitor mode.
In other modes buffering the output helps with large outputs
like route dumps.

Move the fflush() into rtnl_listen() which is the common code
used by all monitoring subcommands. This also ensures any future
monitor subcommand gets correct flushing behavior automatically.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>