FUSE extattrs: fix issue when neither uio nor size were not passed to VOP_*.

The requested size was returned incorrectly in case uio == NULL from listextattr because the
nameprefix/name conversion was not applied.
Also, make a_size/uio returning logic more unified with other filesystems.

Reviewed by: cem, pfg
MFC after: 2 weeks

Differential Revision: https://reviews.freebsd.org/D13528