Use __linux__ instead of LINUX_DATAPATH in C code.

Use __linux__ instead of LINUX_DATAPATH in C code.

The LINUX_DATAPATH C preprocessor symbol was originally meant to be used as
a signal for whether the Linux datapath module could be used, but it was
used as a proxy for a lot of other stuff that is really just Linux
specific.  This commit switches all of these users to just test for
__linux__, which is more straightforward and should have the same result.

CC: Luigi Rizzo <rizzo@iet.unipi.it>
Signed-off-by: Ben Pfaff <blp@nicira.com>