SUNRPC: Use unsigned loop and array index in svc_init_buffer()

Clean up: Suppress a harmless compiler warning.

Index rq_pages[] with an unsigned type.  Make "pages" unsigned as well,
as it never represents a value less than zero.

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