[POWERPC] Correct the MAX_CONTEXT definition

When we increased the address space per process to 2^44 bytes, the
number of contexts that we could actually use reduced, but we forgot
to decrease the MAX_CONTEXT definition.  (Fortunately this would only
cause problems if we actually had more than 512k user processes
running.)  This patch corrects the definition.

Signed-off-by: Paul Mackerras <paulus@samba.org>