FS-Cache: Out of line fscache_operation_init()

Out of line fscache_operation_init() so that it can access internal FS-Cache
features, such as stats, in a later commit.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Steve Dickson <steved@redhat.com>
Acked-by: Jeff Layton <jeff.layton@primarydata.com>