Linux: Actually use freezer compatibility func

Linux: Actually use freezer compatibility func

We were calling try_to_sleep, rather than afs_try_to_sleep. Whilst
try_to_sleep is present in all modern Linux kernels, on some older
systems we need to fall back to our own implementation, which is
what the afs_try_to_sleep function should do, but it can only do so
if we call it.

Change-Id: I900e50cf2754535e676d582bd3da82f1833bde52
Reviewed-on: http://gerrit.openafs.org/2384
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>