fscrypt: clear keyring before calling key_put()

Now that the key quotas are handled immediately on key_put() instead of
being postponed to the key management garbage collection worker, a call
to keyring_clear() is all that is required in fscrypt_put_master_key()
so that the keyring clean-up is also done synchronously.  This patch
should fix the fstest generic/581 flakiness.

Signed-off-by: Luis Henriques <lhenriques@suse.de>
Link: https://lore.kernel.org/r/20240206101619.8083-1-lhenriques@suse.de


[ebiggers: added comment]
Signed-off-by: Eric Biggers <ebiggers@google.com>