samples/bpf: xdpsock: Remove forward declaration of ip_fast_csum()

There is a forward declaration of ip_fast_csum() just before its
implementation, remove the unneeded forward declaration.

While at it mark the implementation as static inline.

Signed-off-by: Niklas Söderlund <niklas.soderlund@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Louis Peens <louis.peens@corigine.com>
Link: https://lore.kernel.org/bpf/20210806122855.26115-3-simon.horman@corigine.com

samples/bpf/xdpsock_user.c

@@ -651,15 +651,13 @@ out:
 	return result;
 }
 
-__sum16 ip_fast_csum(const void *iph, unsigned int ihl);
-
 /*
  *	This is a version of ip_compute_csum() optimized for IP headers,
  *	which always checksum on 4 octet boundaries.
  *	This function code has been taken from
  *	Linux kernel lib/checksum.c
  */
-__sum16 ip_fast_csum(const void *iph, unsigned int ihl)
+static inline __sum16 ip_fast_csum(const void *iph, unsigned int ihl)
 {
 	return (__sum16)~do_csum(iph, ihl * 4);
 }