mptcp: fix sometimes-uninitialized warning

Nathan reported this issue:

  $ make -skj"$(nproc)" ARCH=x86_64 LLVM=1 LLVM_IAS=1 mrproper allmodconfig net/mptcp/subflow.o
  net/mptcp/subflow.c:877:6: warning: variable 'incr' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
    877 |         if (WARN_ON_ONCE(offset > skb->len))
        |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  include/asm-generic/bug.h:101:33: note: expanded from macro 'WARN_ON_ONCE'
    101 | #define WARN_ON_ONCE(condition) ({                              \
        |                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    102 |         int __ret_warn_on = !!(condition);                      \
        |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    103 |         if (unlikely(__ret_warn_on))                            \
        |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    104 |                 __WARN_FLAGS(BUGFLAG_ONCE |                     \
        |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    105 |                              BUGFLAG_TAINT(TAINT_WARN));        \
        |                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    106 |         unlikely(__ret_warn_on);                                \
        |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    107 | })
        | ~~
  net/mptcp/subflow.c:893:6: note: uninitialized use occurs here
    893 |         if (incr)
        |             ^~~~
  net/mptcp/subflow.c:877:2: note: remove the 'if' if its condition is always false
    877 |         if (WARN_ON_ONCE(offset > skb->len))
        |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    878 |                 goto out;
        |                 ~~~~~~~~
  net/mptcp/subflow.c:874:18: note: initialize the variable 'incr' to silence this warning
    874 |         u32 offset, incr, avail_len;
        |                         ^
        |                          = 0
  1 warning generated.

As mentioned by Nathan, this issue is present because 5.10 does not
include commit ea4ca586 ("mptcp: refine MPTCP-level ack scheduling"),
which removed the use of 'incr' in the error path added by this change.
This other commit does not really look suitable for stable, hence this
dedicated patch for 5.10.

Fixes: e93fa44f ("mptcp: fix duplicate data handling")
Reported-by: Nathan Chancellor <nathan@kernel.org>
Closes: https://lore.kernel.org/20240928175524.GA1713144@thelio-3990X


Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>