libceph: Partially revert changes to support MSG_SPLICE_PAGES

Fix the mishandling of MSG_DONTWAIT and also reinstates the per-page
checking of the source pages (which might have come from a DIO write by
userspace) by partially reverting the changes to support MSG_SPLICE_PAGES
and doing things a little differently.  In messenger_v1:

 (1) The ceph_tcp_sendpage() is resurrected and the callers reverted to use
     that.

 (2) The callers now pass MSG_MORE unconditionally.  Previously, they were
     passing in MSG_MORE|MSG_SENDPAGE_NOTLAST and then degrading that to
     just MSG_MORE on the last call to ->sendpage().

 (3) Make ceph_tcp_sendpage() a wrapper around sendmsg() rather than
     sendpage(), setting MSG_SPLICE_PAGES if sendpage_ok() returns true on
     the page.

In messenger_v2:

 (4) Bring back do_try_sendpage() and make the callers use that.

 (5) Make do_try_sendpage() use sendmsg() for both cases and set
     MSG_SPLICE_PAGES if sendpage_ok() is set.

Fixes: 40a8c17a ("ceph: Use sendmsg(MSG_SPLICE_PAGES) rather than sendpage")
Fixes: fa094cca

 ("ceph: Use sendmsg(MSG_SPLICE_PAGES) rather than sendpage()")
Reported-by: Ilya Dryomov <idryomov@gmail.com>
Link: https://lore.kernel.org/r/CAOi1vP9vjLfk3W+AJFeexC93jqPaPUn2dD_4NrzxwoZTbYfOnw@mail.gmail.com/
Link: https://lore.kernel.org/r/CAOi1vP_Bn918j24S94MuGyn+Gxk212btw7yWeDrRcW1U8pc_BA@mail.gmail.com/
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Xiubo Li <xiubli@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: Jens Axboe <axboe@kernel.dk>
cc: Matthew Wilcox <willy@infradead.org>
Link: https://lore.kernel.org/r/3101881.1687801973@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/3111635.1687813501@warthog.procyon.org.uk/ # v2
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Link: https://lore.kernel.org/r/3199652.1687873788@warthog.procyon.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>