s3:smbd: Add functions calc_max_read_pdu()/calc_read_size() to work out the length...

s3:smbd: Add functions calc_max_read_pdu()/calc_read_size() to work out the length we should return.

LARGE_READX test shows it's always safe to return a short read.
Windows does so. Do the calculations to return what will fit
in a read depending on what the client negotiated.

Signed-off-by: Jeremy Allison <jra@samba.org>
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>