seastar: update seastar submodule

to pick up recent fixes in file io

Signed-off-by: Kefu Chai <kchai@redhat.com>

diff --git a/src/seastar b/src/seastar
index d2c0df06d1fd92734d093cea04a13f820c0dbd0a..afafbaa8d43627fffa00541ac26a31904d0268ab 160000 (submodule)
--- a/src/seastar
+++ b/src/seastar
@@ -1 +1 @@
-Subproject commit d2c0df06d1fd92734d093cea04a13f820c0dbd0a
+Subproject commit afafbaa8d43627fffa00541ac26a31904d0268ab