restore(8): Handle extended attribute names correctly

UFS2 extended attribute names are not NUL-terminated. Handle
appropriately.

Correct the EXTATTR_BASE_LENGTH() macro, which handled ea_namelength ==
one (mod eight) extended attributes incorrectly.

PR: 216127
Reported by: dewayne at heuristicsystems.com.au
Reviewed by: kib@
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D9208