ibmvnic: set ltb->buff to NULL after freeing

ibmvnic: set ltb->buff to NULL after freeing

[ Upstream commit 552a33729f1a7cc5115d0752064fe9abd6e3e336 ]

free_long_term_buff() checks ltb->buff to decide whether we have a long
term buffer to free. So set ltb->buff to NULL afer freeing. While here,
also clear ->map_id, fix up some coding style and log an error.

Fixes: 9c4eaabd1bb39 ("Check CRQ command return codes")
Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>