BUG/MEDIUM: fd: prevent memory waste in fdtab array

In 97ea9c49f1 ("BUG/MEDIUM: fd: always align fdtab[] to 64 bytes"), the
patch doesn't do what the message says. The intent was only to align the
base fdtab addr on 64 bytes so that all fdtab entries are aligned and thus
don't share the same cache line. For that, fdtab pointer is adjusted from
fdtab_addr (unaligned) address after it is allocated. Thus, all we need
is an extra 64 bytes in the fdtab_addr array for the aligment. Because
we use calloc() to perform the allocation, a dumb mistake was made: the
'+64' was added on <size> calloc argument, which means EACH fdtab entry
is allocated with 64 extra bytes.

Given that a single fdtab entry is 64 bytes, since 97ea9c49f1 each fdtab
entry now takes 128 bytes! We doubled fdtab memory consumption.

To give you an idea, on my laptop, when looking at memory consumption
using 'ps -p `pidof haproxy` -o size' right after starting haproxy
process with default settings (no maxsock enforced):

before 97ea9c49f1:
  -> 118440 (KB, ~= 118MB)

after 97ea9c49f1:
  -> 183976 (KB, ~= 184MB)

To fix this, use calloc with 1 <nmemb> and manually provide the size with
<size> as we would do if we used malloc(). With this patch, we're back to
pre-97ea9c49f1 for fdtab  memory consumption (with 64 extra bytes the
whole array, which is insignificant).

It should be backported to all stable versions.

src/fd.c

@@ -1158,7 +1158,7 @@ int init_pollers()
 	int p;
 	struct poller *bp;
 
-	if ((fdtab_addr = calloc(global.maxsock, sizeof(*fdtab) + 64)) == NULL) {
+	if ((fdtab_addr = calloc(1, global.maxsock * sizeof(*fdtab) + 64)) == NULL) {
 		ha_alert("Not enough memory to allocate %d entries for fdtab!\n", global.maxsock);
 		goto fail_tab;
 	}