netfs: Make netfs_advance_write() return size_t

netfs_advance_write() calculates the amount of data it's attaching to a
stream with size_t, but then returns this as an int.  Switch the return
value to size_t for consistency.

Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/20241216204124.3752367-7-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
Signed-off-by: Christian Brauner <brauner@kernel.org>