s3: VFS: vfs_aio_pthread: Make aio opens safe against connection teardown.

s3: VFS: vfs_aio_pthread: Make aio opens safe against connection teardown.

Allocate state off fsp->conn, not NULL, and add a destructor
that catches deallocation of conn which happens
on connection shutdown or force close.

Note - We don't allocate off fsp as the passed in
fsp will get freed once we return EINPROGRESS/NT_STATUS_MORE_PROCESSING_REQUIRED.
A new fsp pointer gets allocated on every re-run of the
open code path.

The destructor allows us to NULL out the saved conn struct pointer
when conn is deallocated so we know not to access deallocated memory.
This matches the async teardown code changes for bug #14301
in pread/pwrite/fsync vfs_default.c and vfs_glusterfs.c

state is still correctly deallocated in all code
paths so no memory leaks.

This allows us to safely complete when the openat()
returns and then return the error NT_STATUS_NETWORK_NAME_DELETED
to the client open request.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14301

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>