selftests/powerpc/papr-vpd: Fix missing variable initialization

The "close handle without consuming VPD" testcase has inconsistent
results because it fails to initialize the location code object it
passes to ioctl() to create a VPD handle. Initialize the location code
to the empty string as intended.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Fixes: 9118c5d32bdd ("powerpc/selftests: Add test for papr-vpd")
Reported-by: Geetika Moolchandani <geetika@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240404-papr-vpd-test-uninit-lc-v2-1-37bff46c65a5@linux.ibm.com

selftests/powerpc/papr_vpd: Check devfd before get_system_loc_code()

Calling get_system_loc_code before checking devfd and errno fails the test
when the device is not available, the expected behaviour is a SKIP.

Change the order of 'SKIP_IF_MSG' to correctly SKIP when the /dev/
papr-vpd device is not available.

Test output before:
Test FAILED on line 271

Test output after:
[SKIP] Test skipped on line 266: /dev/papr-vpd not present

Signed-off-by: R Nageswara Sastry <rnsastry@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240131130859.14968-1-rnsastry@linux.ibm.com

powerpc/selftests: Add test for papr-vpd

Add selftests for /dev/papr-vpd, exercising the common expected use
cases:

* Retrieve all VPD by passing an empty location code.
* Retrieve the "system VPD" by passing a location code derived from DT
root node properties, as done by the vpdupdate command.

The tests also verify that certain intended properties of the driver
hold:

* Passing an unterminated location code to PAPR_VPD_CREATE_HANDLE gets
EINVAL.
* Passing a NULL location code pointer to PAPR_VPD_CREATE_HANDLE gets
EFAULT.
* Closing the device node without first issuing a
PAPR_VPD_CREATE_HANDLE command to it succeeds.
* Releasing a handle without first consuming any data from it
succeeds.
* Re-reading the contents of a handle returns the same data as the
first time.

Some minimal validation of the returned data is performed.

The tests are skipped on systems where the papr-vpd driver does not
initialize, making this useful only on PowerVM LPARs at this point.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231212-papr-sys_rtas-vs-lockdown-v6-12-e9eafd0c8c6c@linux.ibm.com