netlink: Handle ifhwioctl(SIOCSIFMTU) failure
Print out error message if setting MTU fails when modifying interface
using netlink.
Reviewed by: saheed, melifaro, mckusick (mentor)
Approved by: saheed, melifaro, mckusick (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52132