asm-generic headers: Allow yet more arch overrides in checksum.h

arches can have more efficient implementation of these routines

Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

lib/checksum.c

@@ -102,6 +102,7 @@ out:
 }
 #endif
 
+#ifndef ip_fast_csum
 /*
  *	This is a version of ip_compute_csum() optimized for IP headers,
  *	which always checksum on 4 octet boundaries.
@@ -111,6 +112,7 @@ __sum16 ip_fast_csum(const void *iph, unsigned int ihl)
 	return (__force __sum16)~do_csum(iph, ihl*4);
 }
 EXPORT_SYMBOL(ip_fast_csum);
+#endif
 
 /*
  * computes the checksum of a memory block at buff, length len,