sunrpc/cache: remove the exporting of cache_seq_next

The function cache_seq_next is declared static and marked
EXPORT_SYMBOL_GPL, which is at best an odd combination. Because the
function is not used outside of the net/sunrpc/cache.c file it is
defined in, this commit removes the EXPORT_SYMBOL_GPL() marking.

Fixes: d48cf356 ("SUNRPC: Remove non-RCU protected lookup")
Signed-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>