vfs_default: pass in state as the callback data to the subreq

vfs_default: pass in state as the callback data to the subreq

Find the req we're finishing off by looking inside the state.  In a shutdown
close the caller calls talloc_free(req), so we can't access it directly as
callback data.

The next commit will NULL out the state->req pointer when a caller calls
talloc_free(req), and the request is still in flight.

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

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>