smbd: let switch_message() only call chdir_current_service() for SMBtdis/SMBexit

smbd: let switch_message() only call chdir_current_service() for SMBtdis/SMBexit

These are the two opcodes with DO_CHDIR, we don't want the
set_current_case_sensitive() logic for them,
so we don't need the full set_current_service() anymore.

The AS_USER case is already handled before, set_current_case_sensitive()
is called directly before change_to_user(), which already
calls chdir_current_service().

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>