Linux: Tidy up freezer code

Linux: Tidy up freezer code

Linux now provides try_to_freeze(), which can be used to replace
all of our refrigerator code. This has been in the tree for a long
time, and actually predates many of the changes we've been modifying
our code to work with. So, use try_to_freeze wherever we can, and keep
a simpler compatibility function for kernels which are too old to have
it.

Change-Id: Iec50f83382f46d2b17b43da815a75755ea916bc6
Reviewed-on: http://gerrit.openafs.org/1845
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>