PCI: Return ~0 data on pciconfig_read() CAP_SYS_ADMIN failure

PCI: Return ~0 data on pciconfig_read() CAP_SYS_ADMIN failure

The pciconfig_read() syscall reads PCI configuration space using
hardware-dependent config accessors.

If the read fails on PCI, most accessors don't return an error; they
pretend the read was successful and got ~0 data from the device, so the
syscall returns success with ~0 data in the buffer.

When the accessor does return an error, pciconfig_read() normally fills the
user's buffer with ~0 and returns an error in errno.  But after
e4585da22ad0 ("pci syscall.c: Switch to refcounting API"), we don't fill
the buffer with ~0 for the EPERM "user lacks CAP_SYS_ADMIN" error.

Userspace may rely on the ~0 data to detect errors, but after e4585da22ad0,
that would not detect CAP_SYS_ADMIN errors.

Restore the original behaviour of filling the buffer with ~0 when the
CAP_SYS_ADMIN check fails.

[bhelgaas: commit log, fold in Nathan's fix
https://lore.kernel.org/r/20210803200836.500658-1-nathan@kernel.org]
Fixes: e4585da22ad0 ("pci syscall.c: Switch to refcounting API")
Link: https://lore.kernel.org/r/20210729233755.1509616-1-kw@linux.com
Signed-off-by: Krzysztof Wilczyński <kw@linux.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: stable@vger.kernel.org