Unbreak building on Linux without sendfile64() support.

Unbreak building on Linux without sendfile64() support.

It was broken in 8e446a2daf48 when the NGX_SENDFILE_LIMIT constant was added
to ngx_linux_sendfile_chain.c having the same name as already defined one in
ngx_linux_config.h.

The newer is needed to overcome a bug in old Linux kernels by limiting the
number of bytes to send per sendfile() syscall.  The older is used with
sendfile() on ancient kernels that works with 32-bit offsets only.

One of these renamed to NGX_SENDFILE_MAXSIZE.