nvme: keyring: fix conditional compilation

The keyring and auth functions can be called from both the host and
the target side and are controlled by Kconfig options for each of the
combinations, but the declarations are controlled by #ifdef checks
on the shared Kconfig symbols.

This leads to link failures in combinations where one of the frontends
is built-in and the other one is a module, and the keyring code
ends up in a module that is not reachable from the builtin code:

ld: drivers/nvme/host/core.o: in function `nvme_core_exit':
core.c:(.exit.text+0x4): undefined reference to `nvme_keyring_exit'
ld: drivers/nvme/host/core.o: in function `nvme_core_init':
core.c:(.init.text+0x94): undefined reference to `nvme_keyring_init

ld: drivers/nvme/host/tcp.o: in function `nvme_tcp_setup_ctrl':
tcp.c:(.text+0x4c18): undefined reference to `nvme_tls_psk_default'

Address this by moving nvme_keyring_init()/nvme_keyring_exit() into
module init/exit functions for the keyring module.

Fixes: be8e82ca

 ("nvme-tcp: enable TLS handshake upcall")
Signed-off-by: Hannes Reinecke <hare@suse.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>