Make sure the mapping_lock also protects access to the various address_space
pointers used for tearing down the ptes on a spu context switch.

Because unmap_mapping_range can sleep we need to turn mapping_lock from
a spinlock into a sleeping mutex.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NArnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: NJeremy Kerr <jk@ozlabs.org>
Signed-off-by: NPaul Mackerras <paulus@samba.org>