Documentation: Add missing documentation for EXPORT_OP flags

    This flag causes nfsd to close any open files for this inode _before_

    calling into the vfs to do an unlink or a rename that would replace

    an existing file.

  EXPORT_OP_REMOTE_FS - Backing storage for this filesystem is remote

    PF_LOCAL_THROTTLE exists for loopback NFSD, where a thread needs to

    write to one bdi (the final bdi) in order to free up writes queued

    to another bdi (the client bdi). Such threads get a private balance

    of dirty pages so that dirty pages for the client bdi do not imact

    the daemon writing to the final bdi. For filesystems whose durable

    storage is not local (such as exported NFS filesystems), this

    constraint has negative consequences. EXPORT_OP_REMOTE_FS enables

    an export to disable writeback throttling.

  EXPORT_OP_NOATOMIC_ATTR - Filesystem does not update attributes atomically

    EXPORT_OP_NOATOMIC_ATTR indicates that the exported filesystem

    cannot provide the semantics required by the "atomic" boolean in

    NFSv4's change_info4. This boolean indicates to a client whether the

    returned before and after change attributes were obtained atomically

    with the respect to the requested metadata operation (UNLINK,

    OPEN/CREATE, MKDIR, etc).

  EXPORT_OP_FLUSH_ON_CLOSE - Filesystem flushes file data on close(2)

    On most filesystems, inodes can remain under writeback after the

    file is closed. NFSD relies on client activity or local flusher

    threads to handle writeback. Certain filesystems, such as NFS, flush

    all of an inode's dirty data on last close. Exports that behave this

    way should set EXPORT_OP_FLUSH_ON_CLOSE so that NFSD knows to skip

    waiting for writeback when closing such files.