fpm: Allow max fpm message size to float based on ecmp

Currently the max message size is 4k.  With a 256 way
ecmp FRR is seeing message sizes that are in the
6k size.  There is desire to allow this to increase as
well to 512.  Since the multipath size directly effects
how big the message may be when sending the routes ecmp
let's give a bit of headroom for this value when compiling
FRR at greater sizes.  Additionally since we know not everyone
is using such large ecmp, allow them to build as appropriate
for their use cases.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>

fpm/fpm.h

@@ -65,7 +65,7 @@
 /*
  * Largest message that can be sent to or received from the FPM.
  */
-#define FPM_MAX_MSG_LEN 4096
+#define FPM_MAX_MSG_LEN MAX(MULTIPATH_NUM * 32, 4096)
 
 #ifdef __SUNPRO_C
 #pragma pack(1)