docs: get rid of :c:type explicit declarations for structs

The :c:type:`foo` only works properly with structs before
Sphinx 3.x.

On Sphinx 3.x, structs should now be declared using the
.. c:struct, and referenced via :c:struct tag.

As we now have the automarkup.py macro, that automatically
convert:
	struct foo

into cross-references, let's get rid of that, solving
several warnings when building docs with Sphinx 3.x.

Reviewed-by: André Almeida <andrealmeid@collabora.com> # blk-mq.rst
Reviewed-by: Takashi Iwai <tiwai@suse.de> # sound
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/vm/ksm.rst

@@ -26,7 +26,7 @@ tree.
 
 If a KSM page is shared between less than ``max_page_sharing`` VMAs,
 the node of the stable tree that represents such KSM page points to a
-list of :c:type:`struct rmap_item` and the ``page->mapping`` of the
+list of struct rmap_item and the ``page->mapping`` of the
 KSM page points to the stable tree node.
 
 When the sharing passes this threshold, KSM adds a second dimension to

Documentation/vm/memory-model.rst

@@ -24,7 +24,7 @@ whether it is possible to manually override that default.
    although it is still in use by several architectures.
 
 All the memory models track the status of physical page frames using
-:c:type:`struct page` arranged in one or more arrays.
+struct page arranged in one or more arrays.
 
 Regardless of the selected memory model, there exists one-to-one
 mapping between the physical page frame number (PFN) and the
@@ -111,7 +111,7 @@ maps for non-volatile memory devices and deferred initialization of
 the memory map for larger systems.
 
 The SPARSEMEM model presents the physical memory as a collection of
-sections. A section is represented with :c:type:`struct mem_section`
+sections. A section is represented with struct mem_section
 that contains `section_mem_map` that is, logically, a pointer to an
 array of struct pages. However, it is stored with some other magic
 that aids the sections management. The section size and maximal number
@@ -172,7 +172,7 @@ management.
 
 The virtually mapped memory map allows storing `struct page` objects
 for persistent memory devices in pre-allocated storage on those
-devices. This storage is represented with :c:type:`struct vmem_altmap`
+devices. This storage is represented with struct vmem_altmap
 that is eventually passed to vmemmap_populate() through a long chain
 of function calls. The vmemmap_populate() implementation may use the
 `vmem_altmap` along with :c:func:`vmemmap_alloc_block_buf` helper to