Ensure MIN/MAX are defined in userspace builds of rx

The include of <sys/param.h> was removed from rx_packet.h on
Linux 2.6 and later to fix kernel builds with 3.7, which doesn't
have that header in kernel space.  However, while kernel space
always provides MIN/MAX defines, userspace relied on the header.
On at least powerpc, no other include chain includes sys/param.h,
so MIN/MAX were left undefined.

Fix this by only skipping the include of <sys/param.h> on Linux
if building in kernel mode.

Change-Id: Icd2edd645ef4d18d626de8ce8b81ac07f37b1f21
Reviewed-on: http://gerrit.openafs.org/8870
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>