nvme: common: make keyring and auth separate modules

When only the keyring module is included but auth is not, modpost
complains about the lack of a module license tag:

ERROR: modpost: missing MODULE_LICENSE() in drivers/nvme/common/nvme-common.o

Address this by making both modules buildable standalone,
removing the now unnecessary CONFIG_NVME_COMMON symbol
in the process.

Also, now that NVME_KEYRING config symbol can be either a module or
built-in, the stubs need to check for '#if IS_ENABLED' rather than a
simple '#ifdef'.

Fixes: 9d77eb52

 ("nvme-keyring: register '.nvme' keyring")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>