sctp: allow setting path_maxrxt independent of SPP_PMTUD_ENABLE

Since draft-ietf-tsvwg-sctpsocket-15.txt, setting the
SPP_MTUD_ENABLE flag when changing pathmaxrxt via the
SCTP_PEER_ADDR_PARAMS setsockopt is not required any
longer.

Signed-off-by: Andrei Pelinescu-Onciul <andrei@iptel.org>
Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>