SUNRPC: drop pointless static qualifier in xdr_get_next_encode_buffer()

There is no need to have the '__be32 *p' variable static since new value
always be assigned before use it.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Cc: stable@vger.kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>