fsqual: stop causing memory leak error on LeakSanitizer

When we enable the sanitizer, we get following error while running
iotune:

==86505==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 4096 byte(s) in 1 object(s) allocated from:
    #0 0x5701b8 in aligned_alloc (/home/syuu/seastar.2/build/sanitize/apps/iotune/iotune+0x5701b8) (BuildId: 411f9852d64ed8982d5b33d02489b5932d92b8b7)
    #1 0x6d0813 in seastar::filesystem_has_good_aio_support(seastar::basic_sstring<char, unsigned int, 15u, true>, bool) /home/syuu/seastar.2/src/core/fsqual.cc:74:16
    #2 0x5bcd0d in main::$_0::operator()() const::'lambda'()::operator()() const /home/syuu/seastar.2/apps/iotune/iotune.cc:742:21
    #3 0x5bb1f1 in seastar::future<int> seastar::futurize<int>::apply<main::$_0::operator()() const::'lambda'()>(main::$_0::operator()() const::'lambda'()&&, std::tuple<>&&) /home/syuu/seastar.2/include/seastar/core/future.hh:2118:28
    #4 0x5bb039 in seastar::futurize<std::invoke_result<main::$_0::operator()() const::'lambda'()>::type>::type seastar::async<main::$_0::operator()() const::'lambda'()>(seastar::thread_attributes, main::$_0::operator()() const::'lambda'()&&)::'lambda'()::operator()() const /home/syuu/seastar.2/include/seastar/core/thread.hh:258:13
    #5 0x5bb039 in seastar::noncopyable_function<void ()>::direct_vtable_for<seastar::futurize<std::invoke_result<main::$_0::operator()() const::'lambda'()>::type>::type seastar::async<main::$_0::operator()() const::'lambda'()>(seastar::thread_attributes, main::$_0::operator()() const::'lambda'()&&)::'lambda'()>::call(seastar::noncopyable_function<void ()> const*) /home/syuu/seastar.2/include/seastar/util/noncopyable_function.hh:124:20
    #6 0x8e0a77 in seastar::thread_context::main() /home/syuu/seastar.2/src/core/thread.cc:299:9
    #7 0x7f30ff8547bf  (/lib64/libc.so.6+0x547bf) (BuildId: 85c438f4ff93e21675ff174371c9c583dca00b2c)

SUMMARY: AddressSanitizer: 4096 byte(s) leaked in 1 allocation(s).

This is because we don't free buffer which allocated at filesystem_has_good_aio_support(), we should free it to avoid such error.

And this is needed to test Scylla machine image with debug mode binary,
since it tries to run iotune with the sanitizer and fails.

Closes #1284

src/core/fsqual.cc

@@ -72,6 +72,7 @@ bool filesystem_has_good_aio_support(sstring directory, bool verbose) {
     auto bufsize = 4096;
     auto ctxsw = 0;
     auto buf = aligned_alloc(4096, 4096);
+    auto del = defer([&] () noexcept { ::free(buf); });
     for (int i = 0; i < nr; ++i) {
         struct iocb cmd;
         cmd = make_write_iocb(fd.get(), bufsize*i, buf, bufsize);