docs: exportfs: Use source code struct documentation

A file system implementation declares that instances of the filesystem

are exportable by setting the s_export_op field in the struct

super_block.  This field must point to a "struct export_operations"

struct which has the following members:

  encode_fh (mandatory)

    Takes a dentry and creates a filehandle fragment which may later be used

    to find or create a dentry for the same object.

  fh_to_dentry (mandatory)

    Given a filehandle fragment, this should find the implied object and

    create a dentry for it (possibly with d_obtain_alias).

  fh_to_parent (optional but strongly recommended)

    Given a filehandle fragment, this should find the parent of the

    implied object and create a dentry for it (possibly with

    d_obtain_alias).  May fail if the filehandle fragment is too small.

  get_parent (optional but strongly recommended)

    When given a dentry for a directory, this should return  a dentry for

    the parent.  Quite possibly the parent dentry will have been allocated

    by d_alloc_anon.  The default get_parent function just returns an error

    so any filehandle lookup that requires finding a parent will fail.

    ->lookup("..") is *not* used as a default as it can leave ".." entries

    in the dcache which are too messy to work with.

  get_name (optional)

    When given a parent dentry and a child dentry, this should find a name

    in the directory identified by the parent dentry, which leads to the

    object identified by the child dentry.  If no get_name function is

    supplied, a default implementation is provided which uses vfs_readdir

    to find potential names, and matches inode numbers to find the correct

    match.

  flags

    Some filesystems may need to be handled differently than others. The

    export_operations struct also includes a flags field that allows the

    filesystem to communicate such information to nfsd. See the Export

    Operations Flags section below for more explanation.

super_block.  This field must point to a struct export_operations

which has the following members:

.. kernel-doc:: include/linux/exportfs.h

   :identifiers: struct export_operations

A filehandle fragment consists of an array of 1 or more 4byte words,

together with a one byte "type".