Linux: fix statfs configure test

Linux: fix statfs configure test

The change to the statfs configure test that was made for 2.6.36
broke the test for older kernels.  The new test is based on a call,
and that will generate a warning but not an error when the arguments
don't match the prototype.

Take another tack, and revert to the old style test, but with the
simple_statfs function instead of vfs_statfs.

Change-Id: Ic4baa24e2497f43791e069ce2c5c87fa21813f9f
Reviewed-on: http://gerrit.openafs.org/2975
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>