The cookie is not used at all, remove it and update the usage in io.c
and afs/write.c (which is the only user outside of fscache currently)
at the same time.

[DH: Amended the documentation also]
Signed-off-by: Yue Hu <huyue2@coolpad.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-cachefs@redhat.com
Link: https://listman.redhat.com/archives/linux-cachefs/2022-April/006659.html