Merge branch 'mptcp-implement-read_sock-and-splice_read'

Matthieu Baerts says:

====================
mptcp: implement .read_sock and .splice_read

This series is a preparation work for future in-kernel MPTCP sockets
usage. Here, two interfaces are implemented: read_sock and splice_read.
As a result of this series, splice() with MPTCP sockets -- which was
already supported -- is now improved.

- Patches 1-2: .read_sock implementation

- Patches 3-4: .splice_read implementation

- Patches 5-6: validate splice() support with MPTCP sockets.
====================

Link: https://patch.msgid.link/20260130-net-next-mptcp-splice-v2-0-31332ba70d7f@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>