seastar: pickup the latest seastar

for couple improvements

* dbf7b6b0: add fast path for small writes
* 2f357317: IPv6 support

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

diff --git a/src/seastar b/src/seastar
index 8a1e3ceb94795def620b47daaa8341b2015f67da..23c9610a820dc2b85fcb97f0579335960f16cb4d 160000 (submodule)
--- a/src/seastar
+++ b/src/seastar
@@ -1 +1 @@
-Subproject commit 8a1e3ceb94795def620b47daaa8341b2015f67da
+Subproject commit 23c9610a820dc2b85fcb97f0579335960f16cb4d