powerpc/pseries: Initialise nvram_pstore_info's buf_lock

nvram_pstore_info's buf_lock is not initialized before registering,
which is clearly incorrect.

It causes some strange behavior when trying to obtain the lock during
kdump process.

On a UP configuration, the console stopped for a couple of seconds, then
"lockup suspected" warning printed out, but then it continued to run.

So try lock fails, and lockup reported, but then arch_spin_lock()
passes.

Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com>
[mpe: Edited changelog]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>