a little improvement for vmalloc

[Zhonglin Zhang <zhonglinzh@mobilesoft.com.cn>]

Hello,

In FUNCTION __vmalloc ,

There is a statement;

if (!size || (size >> PAGE_SHIFT) > num_physpages)
        return NULL;

I think the condition (num_phypages >>PAGE_SHIFT) > num_physpages 
is not very accurate. As we all know, linux kernel and other stuff
occupy some memory,so it is better to express like below, I think. 

if (!size || size > max_vmalloc_size)
	return NULL;

max_vmalloc_size = (num_physpages >> PAGE_SHIFT) - kernel_size          
                             -reserved_space_for_emergence_use


BTW, I would like to know whether there are reserved physical memory for
emergence use.

Thanks in advance!

Milo