SUNRPC: Add svc_rqst_replace_page() API

[ Upstream commit 2f0f88f4 ]

Replacing a page in rq_pages[] requires a get_page(), which is a
bus-locked operation, and a put_page(), which can be even more
costly.

To reduce the cost of replacing a page in rq_pages[], batch the
put_page() operations by collecting "freed" pages in a pagevec,
and then release those pages when the pagevec is full. This
pagevec is also emptied when each RPC completes.

[ cel: adjusted to apply without f6e70aab ]
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>