vfs_extattr: Allow extattr names up to the full max

Extattr names are allowed to be 255 bytes -- not 254 bytes plus trailing
NUL. Provide a 256 buffer so that copyinstr() has room for the trailing
NUL.

Re-enable test for maximal name lengths.

PR: 208965
Reported by: asomers
Reviewed by: asomers
Differential Revision: https://reviews.freebsd.org/D24584