ufs/extattr.h: Fix documentation of ea_name termination

The ea_name string is not nul-terminated. Correct the documentation.

Because the subsequent field is padded to 8 bytes, and the padding is
zeroed, the ea_name string will appear to be nul-terminated whenever the
length isn't exactly one (mod eight).

This was introduced in r167010 (2007).

Additionally, mark the length fields as unsigned. This particularly
matters for the single byte ea_namelength field, which can represent
extended attribute names up to 255 bytes long.

No functional change.

PR: 216127
Reported by: dewayne at heuristicsystems.com.au
Reviewed by: kib@
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D9206