[POWERPC] spufs: spu_find_victim may choose wrong victim

Need to re-check priority after dropping lock.  Otherwise, a
more favored context may be preempted.

Signed-off-by: Luke Browning <lukebr@linux.vnet.ibm.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>