[IA64] sys_mmap doesn't follow posix.1 when parameter len=0

In IA64 kernel, sys_mmap calls do_mmap2 and do_mmap2 returns addr if
len=0, which means the mmap sys call succeeds.

Posix.1 says:
The mmap() function shall fail if:
[EINVAL] The value of len is zero. 

Here is a patch to fix it.

Signed-off-by: Zhang Yanmin <yanmin.zhang@intel.com>
Acked-by: David Mosberger <davidm@hpl.hp.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>