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.

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>
(cherry picked from commit fe2bae1c83a6d050e0afb88bad8fe02beae343e7)

Change-Id: I4b2b123c493ab35cbeff309db7ff58354afc170b
Reviewed-on: http://gerrit.openafs.org/8872
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Paul Smeddle <paul.smeddle@gmail.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>