mac80211: fix race in ieee80211_register_hw()

mac80211: fix race in ieee80211_register_hw()

A race condition leading to a kernel crash is observed during invocation
of ieee80211_register_hw() on a dragonboard410c device having wcn36xx
driver built as a loadable module along with a wifi manager in user-space
waiting for a wifi device (wlanX) to be active.

Sequence diagram for a particular kernel crash scenario:

    user-space  ieee80211_register_hw()  ieee80211_tasklet_handler()
    ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
       |                    |                 |
       |<---phy0----wiphy_register()          |
       |-----iwd if_add---->|                 |
       |                    |<---IRQ----(RX packet)
       |              Kernel crash            |
       |              due to unallocated      |
       |              workqueue.              |
       |                    |                 |
       |       alloc_ordered_workqueue()      |
       |                    |                 |
       |              Misc wiphy init.        |
       |                    |                 |
       |            ieee80211_if_add()        |
       |                    |                 |

As evident from above sequence diagram, this race condition isn't specific
to a particular wifi driver but rather the initialization sequence in
ieee80211_register_hw() needs to be fixed. So re-order the initialization
sequence and the updated sequence diagram would look like:

    user-space  ieee80211_register_hw()  ieee80211_tasklet_handler()
    ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
       |                    |                 |
       |       alloc_ordered_workqueue()      |
       |                    |                 |
       |              Misc wiphy init.        |
       |                    |                 |
       |<---phy0----wiphy_register()          |
       |-----iwd if_add---->|                 |
       |                    |<---IRQ----(RX packet)
       |                    |                 |
       |            ieee80211_if_add()        |
       |                    |                 |

Cc: stable@vger.kernel.org
Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Link: https://lore.kernel.org/r/1586254255-28713-1-git-send-email-sumit.garg@linaro.org
[Johannes: fix rtnl imbalances]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>