netfilter: nf_nat_ftp: remove (*mangle[]) array and functions, use %pI4

These functions merely exist to format a buffer and call
nf_nat_mangle_tcp_packet.

Format the buffer and perform the call in nf_nat_ftp instead.

Use %pI4 for the IP address.

Saves ~600 bytes of text

old:
$ size net/ipv4/netfilter/nf_nat_ftp.o
   text	   data	    bss	    dec	    hex	filename
   2187	    160	    408	   2755	    ac3	net/ipv4/netfilter/nf_nat_ftp.o
new:
$ size net/ipv4/netfilter/nf_nat_ftp.o
   text    data     bss     dec     hex filename
   1532     112     288    1932     78c net/ipv4/netfilter/nf_nat_ftp.o

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>