SUNRPC: Change list definition method

SUNRPC: Change list definition method

The LIST_HEAD macro can both define a linked list and initialize
it in one step. To simplify code, we replace the separate operations
of linked list definition and manual initialization with the LIST_HEAD
macro.

Signed-off-by: Chenguang Zhao <zhaochenguang@kylinos.cn>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>