Get an extended attribute
If size is zero, the size of the value should be sent with fuse_reply_xattr.
If the size is non-zero, and the value fits in the buffer, the value should be sent with fuse_reply_buf.
If the size is too small for the value, the ERANGE error should be sent.
Valid replies: fuse_reply_buf fuse_reply_xattr fuse_reply_err