IB/hfi1: Fix sdma.h tx->num_descs off-by-one errors

Fix three sources of error involving struct sdma_txreq.num_descs.

When _extend_sdma_tx_descs() extends the descriptor array, it uses the
value of tx->num_descs to determine how many existing entries from the
tx's original, internal descriptor array to copy to the newly allocated
one.  As this value was incremented before the call, the copy loop will
access one entry past the internal descriptor array, copying its contents
into the corresponding slot in the new array.

If the call to _extend_sdma_tx_descs() fails, _pad_smda_tx_descs() then
invokes __sdma_tx_clean() which uses the value of tx->num_desc to drive a
loop that unmaps all descriptor entries in use.  As this value was
incremented before the call, the unmap loop will invoke sdma_unmap_desc()
on a descriptor entry whose contents consist of whatever random data was
copied into it during (1), leading to cascading further calls into the
kernel and driver using arbitrary data.

_sdma_close_tx() was using tx->num_descs instead of tx->num_descs - 1.

Fix all of the above by:
- Only increment .num_descs after .descp is extended.
- Use .num_descs - 1 instead of .num_descs for last .descp entry.

Fixes: f4d26d81

 ("staging/rdma/hfi1: Add coalescing support for SDMA TX descriptors")
Link: https://lore.kernel.org/r/167656658879.2223096.10026561343022570690.stgit@awfm-02.cornelisnetworks.com
Signed-off-by: Brendan Cunningham <bcunningham@cornelisnetworks.com>
Signed-off-by: Patrick Kelsey <pat.kelsey@cornelisnetworks.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>