powerpc/booke: Fix handling of pvh_global_lock and pmap lock

ptbl_alloc() is expected to return with the pvh_global_lock and pmap
lock held. However, it will return with them unlocked if nosleep is
specified.

Along with this, fix lock ordering of pvh_global_lock with respect to
the pmap lock in other places.

Differential Revision: https://reviews.freebsd.org/D23692