iavf: fix potential deadlock on allocation failure

In iavf_adminq_task(), if kzalloc() fails to allocate the event.msg_buf,
the function will exit without releasing the adapter->crit_lock.

This is unlikely, but if it happens, the next access to that mutex will
deadlock.

Fix this by moving the unlock to the end of the function, and adding a new
label to allow jumping to the unlock portion of the function exit flow.

Fixes: fc2e6b3b ("iavf: Rework mutexes for better synchronisation")
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Rafal Romanowski <rafal.romanowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>