ceph: remove bogus checks and WARN_ONs from ceph_set_page_dirty

The checks for page->mapping are odd, as set_page_dirty is an
address_space operation, and I don't see where it would be called on a
non-pagecache page.

The warning about the page lock also seems bogus.  The comment over
set_page_dirty() says that it can be called without the page lock in
some rare cases. I don't think we want to warn if that's the case.

Reported-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>