net/smc: fix access to parent of an ib device

The parent of an ib device is used to retrieve the PCI device
attributes. It turns out that there are possible cases when an ib device
has no parent set in the device structure, which may lead to page
faults when trying to access this memory.
Fix that by checking the parent pointer and consolidate the pci device
specific processing in a new function.

Fixes: a3db10ef ("net/smc: Add support for obtaining SMCR device list")
Reported-by:  <syzbot+600fef7c414ee7e2d71b@syzkaller.appspotmail.com>
Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Link: https://lore.kernel.org/r/20201215091058.49354-2-kgraul@linux.ibm.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>