s390/vfio-ap: make sure nib is shared

Since the NIB is visible by HW, KVM and the (PV) guest it needs to be
in non-secure or secure but shared storage. Return code 6 is used to
indicate to a PV guest that its NIB would be on secure, unshared
storage and therefore the NIB address is invalid.

Unfortunately we have no easy way to check if a page is unshared after
vfio_pin_pages() since it will automatically export an unshared page
if the UV pin shared call did not succeed due to a page being in
unshared state.

Therefore we use the fact that UV pinning it a second time is a nop
but trying to pin an exported page is an error (0x102). If we
encounter this error, we do a vfio unpin and import the page again,
since vfio_pin_pages() exported it.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
Acked-by: Halil Pasic <pasic@linux.ibm.com>
Tested-by: Viktor Mihajlovski <mihajlov@linux.ibm.com>
Link: https://lore.kernel.org/r/20230815184333.6554-13-akrowiak@linux.ibm.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>