portals4: reduce unexpected buffer space

Out previous increase consumed too much memory. 8MB should be enough given
the maximum put size and number of headers.
Signed-off-by: Antonio J. Pena <apenya@mcs.anl.gov>

src/mpid/ch3/channels/nemesis/netmod/portals4/ptl_poll.c

@@ -7,7 +7,7 @@
 #include "ptl_impl.h"
 
 #define OVERFLOW_LENGTH (1024*1024)
-#define NUM_OVERFLOW_ME 512
+#define NUM_OVERFLOW_ME 8
 
 static ptl_handle_me_t overflow_me_handle[NUM_OVERFLOW_ME];
 static void *overflow_buf[NUM_OVERFLOW_ME];