datapath: Don't pass __GFP_ZERO to kmalloc on older kernels.

datapath: Don't pass __GFP_ZERO to kmalloc on older kernels.

On new kernels kzalloc() is simply a wrapper around kmalloc with
the addition of the __GFP_ZERO flag.  flex_arrays take advantage
of this by expecting the user to just pass in this flag if they
want the memory to be zeroed.  However, before 2.6.23, kzalloc()
was a function in its own right and kmalloc really didn't like
receiving __GFP_ZERO.  This overrides kmalloc() to intercept the
flags and direct the call to the right function.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>