s3:smbd: let SMB_VFS_GETXATTRAT_SEND() do explicit impersonation

s3:smbd: let SMB_VFS_GETXATTRAT_SEND() do explicit impersonation

SMB_VFS_GETXATTRAT_SEND() gets passed a raw event context and the
default implementation uses that as well a raw threadpool. Impersonation
is done explicitly instead of by the tevent and pthreadpool wrappers.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>