lagg: update capabilites on SIOCSIFMTU

Some NICs might have limited capabilities when Jumbo frames are used.
For exampe some neta interfaces only support TX csum offload when the
packet size is lower than a value specified in DT.
Fix it by re-reading capabilities of children interfaces after MTU
has been successfully changed.

Found by: Jerome Tomczyk <jerome.tomczyk@stormshield.eu>
Reviewed by: jhb
Obtained from: Semihalf
Sponsored by: Stormshield
Differential revision: https://reviews.freebsd.org/D32724