common/buffer_seastar: fix alien threads memory

common/buffer_seastar: fix alien threads memory

The underlying raw_seastar_foreign_ptr::ptr is allocated from seastar.
This ptr is wrapped with seastar::foreign_ptr:
```
/// \c foreign_ptr<> wraps smart pointers -- \ref seastar::shared_ptr<>,
/// or similar, and remembers on what core this happened.
/// When the \c foreign_ptr<> object is destroyed, it sends a message to
/// the original core so that the wrapped object can be safely destroyed.
```

The issue is that once the pointer is de-allocated from an alien thread
it is unable to send a message to the original core.
Fix this issue by making use of seastar::alien integration with non-seastar applications.
In case ~raw_seastar_foreign_ptr() will be called from an alien thread, we will submit *and wait*
for the memory to be released from the origin core.

Fixes: https://tracker.ceph.com/issues/64086
Signed-off-by: Matan Breizman <mbreizma@redhat.com>