BGP: Use proper class in attribute error messages

Most error messages in attribute processing are in rx/decode step and
these use L_REMOTE log class. But there are few that are in tx/export
step and these should use L_ERR log class.

Use tx-specific macro (REJECT()) in tx/export code and rename field
err_withdraw to err_reject in struct bgp_export_state to ensure that
appropriate error reporting macros are called in proper contexts.