Linux 4.7: Follow key_alloc API change

Linux 4.7: Follow key_alloc API change

Linux v4.7-rc1~124^2~2^2^2~9 adds an eighth optional argument
restrict_link.  The same commit adds a KEY_ALLOC_BYPASS_RESTRICTION
macro, which we test so we can avoid adding another configure test.

Change-Id: I83e27b54ba5711124dccaa41de7155be77054f47
Reviewed-on: https://gerrit.openafs.org/12345
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Anders Kaseorg <andersk@mit.edu>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>