switchtec: Fix Spectre v1 vulnerability

switchtec: Fix Spectre v1 vulnerability

p.port can is indirectly controlled by user-space, hence leading to
a potential exploitation of the Spectre variant 1 vulnerability.

This issue was detected with the help of Smatch:

  drivers/pci/switch/switchtec.c:912 ioctl_port_to_pff() warn: potential spectre issue 'pcfg->dsp_pff_inst_id' [r]

Fix this by sanitizing p.port before using it to index
pcfg->dsp_pff_inst_id

Notice that given that speculation windows are large, the policy is to kill
the speculation on the first load and not worry if it can be completed with
a dependent load/store [1].

[1] https://marc.info/?l=linux-kernel&m=152449131114778&w=2

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Logan Gunthorpe <logang@deltatee.com>
Cc: stable@vger.kernel.org