cachefiles: Set the max subreq size for cache writes to MAX_RW_COUNT

Set the maximum size of a subrequest that writes to cachefiles to be
MAX_RW_COUNT so that we don't overrun the maximum write we can make to the
backing filesystem.
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/1599005.1721398742@warthog.procyon.org.uk
cc: Jeff Layton <jlayton@kernel.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/cachefiles/io.c

@@ -630,7 +630,7 @@ static void cachefiles_prepare_write_subreq(struct netfs_io_subrequest *subreq)
 
 	_enter("W=%x[%x] %llx", wreq->debug_id, subreq->debug_index, subreq->start);
 
-	subreq->max_len = ULONG_MAX;
+	subreq->max_len = MAX_RW_COUNT;
 	subreq->max_nr_segs = BIO_MAX_VECS;
 
 	if (!cachefiles_cres_file(cres)) {