Fix #1 for LP: #1795659

Ubuntu-aws-4.15.0-1027.27+

Author: Aurelien Aptel <email address hidden>
Date: Fri Mar 29 10:49:12 2019 +0100

    CIFS: keep FileInfo handle live during oplock break

    In the oplock break handler, writing pending changes from pages puts
    the FileInfo handle. If the refcount reaches zero it closes the handle
    and waits for any oplock break handler to return, thus causing a deadlock.

    To prevent this situation:

    * We add a wait flag to cifsFileInfo_put() to decide whether we should
      wait for running/pending oplock break handlers

    * We keep an additionnal reference of the SMB FileInfo handle so that
      for the rest of the handler putting the handle won't close it.
      - The ref is bumped everytime we queue the handler via the
        cifs_queue_oplock_break() helper.
      - The ref is decremented at the end of the handler

    This bug was triggered by xfstest 464.

    Also important fix to address the various reports of
    oops in smb2_push_mandatory_locks

    Signed-off-by: Aurelien Aptel <email address hidden>
    Signed-off-by: Steve French <email address hidden>
    Reviewed-by: Pavel Shilovsky <email address hidden>
    CC: Stable <email address hidden>
    (cherry picked from commit b98749cac4a695f084a5ff076f4510b23e353ecd)
    Signed-off-by: Dragan Stancevic <email address hidden>