KEYS: Fix key_reject_and_link() race with keyring restriction

In the Linux kernel, the following vulnerability has been resolved:

KEYS: Fix key_reject_and_link() race with keyring restriction

The restrict_link check in key_reject_and_link() is done before taking
the keyring semaphore, racing with a concurrent keyring_restrict()
that installs the restriction while holding the semaphore in write
mode.

Move the check to after __key_link_lock(), so it is done under the
semaphore, matching the pattern used in __key_create_or_update().