s3: VFS: vfs_default. Pass in struct vfswrap_pread_state as the callback data to...

s3: VFS: vfs_default. Pass in struct vfswrap_pread_state as the callback data to the subreq.

Find the req we're finishing off by looking inside vfswrap_pread_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 vfswrap_pread_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: Jeremy Allison <jra@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>