s3: VFS: vfs_default. Protect vfs_fsync_done() from accessing a freed req pointer.

s3: VFS: vfs_default. Protect vfs_fsync_done() from accessing a freed req pointer.

If the fsp is forced closed by a SHUTDOWN_CLOSE whilst the
request is in flight (share forced closed by smbcontrol),
then we set state->req = NULL in the state destructor.

The existing state destructor prevents the state memory
from being freed, so when the thread completes and calls
vfs_fsync_done(), just throw away the result if
state->req == NULL.

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>