Merge branch 'mptcp-miscellaneous-refactoring-and-small-fixes'

Mat Martineau says:

====================
mptcp: Miscellaneous refactoring and small fixes

Patches 1-3 do some refactoring to more consistently handle sock casts,
and to remove some duplicate code. No functional changes.

Patch 4 corrects a variable name in a self test, but does not change
functionality since the same value gets used due to bash's
scoping rules.

Patch 5 rewords a comment.
====================

Link: https://lore.kernel.org/r/20221110232322.125068-1-mathew.j.martineau@linux.intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>