netfs: Export the netfs_sreq tracepoint

Export the netfs_sreq tracepoint so that it can be called directly from
client filesystems/cache backend modules.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org