afs: Make afs_init_request() get a key if not given a file

afs: Make afs_init_request() get a key if not given a file

In a future patch, AFS directory caching will go through netfslib and this
will involve, at times, running on behalf of ->lookup(), which doesn't
provide us with a file from which we can get an authentication key.

If a file isn't provided, make afs_init_request() get a key from the
process's keyrings instead when setting up a read.

Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/20241216204124.3752367-21-dhowells@redhat.com
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Signed-off-by: Christian Brauner <brauner@kernel.org>