ANDROID: Fix gki allmodconfig build errors in mptcp

In commit e93fa44f ("mptcp: fix duplicate data handling"), the logic
could potentially end up testing the variable 'incr' without first
setting it.  clang properly catches this error, but gcc doesn't, and
it's gone unnoticed upstream as well.

Fix this up by initializing the variable, which resolves the build.
Note that for GKI kernels, none of this matters as mptcp is not enabled
for that build.

Fixes: e93fa44f ("mptcp: fix duplicate data handling")
Change-Id: Ia619c1f9fed70f39b29cfdbd3143e0480fd2199d
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>