Update seastar submodule
Update seastar submodule

* seastar 994b4b5a0c...083898a172 (24):
  > Revert "memory: always allocate buf using "malloc" for non reactor"
  > Revert dpdk update to 21.08.
  > tutorial: Fix typos
  > queue: add back template requirement for element type to be nothrow move-constructible
  > Revert "queue: require element type to be nothrow move-constructible"
  > build: add the closing "-Wl,--no-whole-archive" to the ldflags
  > build: add -Wno-error=volatile to CXX_FLAGS
  > build: Include dpdk as a single object in libseastar.a
  > Merge: queue: cleanup exception handling
  > build: drop dpdk-specific machine architecture names
  > reactor: call memory::configure() before initialize dpdk
  > core/loop: parallel_for_each(): make entire function critical alloc section
  > Merge 'scheduling groups: Add compile parameter for setting max scheduling groups count at compile time' from Eliran Sinvani
  > test: coroutines_test: assign spinner lambda to local variable
  > shared_ptr: mark shared_from_this functions noexcept
  > lw_shared_ptr: mark shared_from_this functions noexcept
  > build: update download URL for Boost
  > Merge "build: build with dpdk v21.08" from Kefu
  > cpu_stall_detector: handle wraparounds in Linux perf_event ring buffer
  > entry_point.cc: default-initialize sigaction struct
  > reactor: s/gettid()/syscall(SYS_gettid)/
  > memory: always allocate buf using "malloc" for non reactor
  > Revert "memory: always allocate buf using "malloc" for non reactor"
  > memory: always allocate buf using "malloc" for non reactor

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla