habanalabs: fix race when waiting on encaps signal

habanalabs: fix race when waiting on encaps signal

Scenario:
1. CS which is part of encaps signal has been completed and now
executing kref_put to its encaps signal handle. The refcount of the
handle decremented to 0, and called the encaps signal handle
release function - hl_encaps_handle_do_release.

2. At this point the user starts waiting on the signal, and finds the
encaps signal handle in the handlers list and increment the habdle
refcount to 1.

3. Immediately after, hl_encaps_handle_do_release removed the handle
from the list and free its memory.

4. Wait function using the handle although it has been freed.

This scenario caused the slab area which was previously allocated
for the handle to be poison overwritten which triggered kernel bug
the next time the OS needed to allocate this slab.

Fixed by getting the refcount of the handle only in case it is not
zero.

Signed-off-by: Dani Liberman <dliberman@habana.ai>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>