DARWIN: Make crfree() an inline function

On Darwin 100, we have
   #define crfree(X) kauth_cred_unref(&X)
which expands in one place to
   kauth_cred_unref(&(afs_ucred_t *)avc->linkData)

clang warns about this in -Werror mode, because arguments to the
address-of operator must be lvalues.

To fix this make crfree an inline function, which will then let us do
the indirection required.

Reviewed-on: http://gerrit.openafs.org/7073
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 1d8937b860509fcaabb041bc14faf7aa3023f3c9)

Change-Id: I869caa41884c33e6376d664a745efb0f8aa761d0
Reviewed-on: http://gerrit.openafs.org/10548
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>