[POWERPC] spufs: serialize SLB invalidation against SLB loading

There is a potential race between flushes of the entire SLB in the MFC
and the point where new entries are being established. The problem is
that we might put a ESID entry into the MFC SLB when the VSID entry has
just been cleared by the global flush.

This can be circumvented by holding the register_lock throughout both
the flushing and the creation of SLB entries.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>