docs: filesystems: convert sysfs-tagging.txt to ReST

- Add a SPDX header;
- Adjust document title;
- Some whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/45a01fa5edd5c6ee8fc0754fc74f7ef65a3e5581.1588021877.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/index.rst

@@ -35,6 +35,7 @@ algorithms work.
    seq_file
    sharedsubtree
    sysfs-pci
+   sysfs-tagging
 
    automount-support
 

Documentation/filesystems/sysfs-tagging.rst

@@ -1,5 +1,8 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=============
 Sysfs tagging
--------------
+=============
 
 (Taken almost verbatim from Eric Biederman's netns tagging patch
 commit msg)
@@ -18,25 +21,28 @@ in the directories and applications only see a limited set of
 the network devices.
 
 Each sysfs directory entry may be tagged with a namespace via the
-void *ns member of its kernfs_node.  If a directory entry is tagged,
-then kernfs_node->flags will have a flag between KOBJ_NS_TYPE_NONE
+``void *ns member`` of its ``kernfs_node``.  If a directory entry is tagged,
+then ``kernfs_node->flags`` will have a flag between KOBJ_NS_TYPE_NONE
 and KOBJ_NS_TYPES, and ns will point to the namespace to which it
 belongs.
 
-Each sysfs superblock's kernfs_super_info contains an array void
-*ns[KOBJ_NS_TYPES].  When a task in a tagging namespace
+Each sysfs superblock's kernfs_super_info contains an array
+``void *ns[KOBJ_NS_TYPES]``.  When a task in a tagging namespace
 kobj_nstype first mounts sysfs, a new superblock is created.  It
 will be differentiated from other sysfs mounts by having its
-s_fs_info->ns[kobj_nstype] set to the new namespace.  Note that
+``s_fs_info->ns[kobj_nstype]`` set to the new namespace.  Note that
 through bind mounting and mounts propagation, a task can easily view
 the contents of other namespaces' sysfs mounts.  Therefore, when a
 namespace exits, it will call kobj_ns_exit() to invalidate any
 kernfs_node->ns pointers pointing to it.
 
 Users of this interface:
-- define a type in the kobj_ns_type enumeration.
-- call kobj_ns_type_register() with its kobj_ns_type_operations which has
+
+- define a type in the ``kobj_ns_type`` enumeration.
+- call kobj_ns_type_register() with its ``kobj_ns_type_operations`` which has
+
   - current_ns() which returns current's namespace
   - netlink_ns() which returns a socket's namespace
   - initial_ns() which returns the initial namesapce
+
 - call kobj_ns_exit() when an individual tag is no longer valid