SUNRPC: Use unsigned index when looping over arrays

Clean up: Suppress a harmless compiler warning in the RPC server related
to array indices.

ARRAY_SIZE() returns a size_t, so use unsigned type for a loop index when
looping over arrays.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>