s390/pci: rename lock member in struct zpci_dev

Since this guards only the Function Measurement Block, rename from
generic lock to fmb_lock in preparation to introduce another lock
that guards the state member

Signed-off-by: Gerd Bayer <gbayer@linux.ibm.com>
Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>