file: /proc/sys/net/ipv6/conf/interface/accept_ra_mtu
variable: net.ipv6.conf.interface.accept_ra_mtu
Official reference

Apply the MTU value specified in RA option 5 (RFC4861). If disabled, the MTU specified in the RA will be ignored.

Functional default: enabled if accept_ra is enabled. disabled if accept_ra is disabled.

Nb: per interface setting (where “interface” is the name of your network interface); “all” is a special interface: changes the settings for all interfaces.