SUNRPC: Add svc_rqst_page_release() helper

SUNRPC: Add svc_rqst_page_release() helper

svc_rqst_replace_page() releases displaced pages through a
per-rqst folio batch, but exposes the add-or-flush sequence
directly. svc_tcp_restore_pages() releases displaced pages
individually with put_page().

Introduce svc_rqst_page_release() to encapsulate the
batched release mechanism. Convert svc_rqst_replace_page()
and svc_tcp_restore_pages() to use it. The latter now
benefits from the same batched release that
svc_rqst_replace_page() already uses.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>