seastar: update submodule

Update submodule to branch https://github.com/ceph/seastar/commits/wip-matanb-seastar-july9/

Signed-off-by: Matan Breizman <mbreizma@redhat.com>
(cherry picked from commit 9fddade5a9e59ca642c98f3a4abd8ff43d72c74b)

diff --git a/src/seastar b/src/seastar
index 96a93ba0cf2d1013914aea203488d2c0cccc54c6..e7a2679235aeea4c9635e5202260204f0147a8c2 160000 (submodule)
--- a/src/seastar
+++ b/src/seastar
@@ -1 +1 @@
-Subproject commit 96a93ba0cf2d1013914aea203488d2c0cccc54c6
+Subproject commit e7a2679235aeea4c9635e5202260204f0147a8c2