docs: filesystems: convert mount_api.txt to ReST

- Add a SPDX header;
- Adjust document and section titles;
- Some whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Add table markups;
- Add lists markups;
- Add it to filesystems/index.rst.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/32332c1659a28c22561cb5e64162c959856066b4.1588021877.git.mchehab+huawei@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/index.rst

@@ -30,6 +30,7 @@ algorithms work.
    files
    locks
    mandatory-locking
+   mount_api
 
    automount-support
 

include/linux/fs_context.h

@@ -85,7 +85,7 @@ struct p_log {
  * Superblock creation fills in ->root whereas reconfiguration begins with this
  * already set.
  *
- * See Documentation/filesystems/mount_api.txt
+ * See Documentation/filesystems/mount_api.rst
  */
 struct fs_context {
 	const struct fs_context_operations *ops;

include/linux/lsm_hooks.h

@@ -77,7 +77,7 @@
  *	state.  This is called immediately after commit_creds().
  *
  * Security hooks for mount using fs_context.
- *	[See also Documentation/filesystems/mount_api.txt]
+ *	[See also Documentation/filesystems/mount_api.rst]
  *
  * @fs_context_dup:
  *	Allocate and attach a security structure to sc->security.  This pointer