Remove get_vmalloc_info() wrapper

Remove get_vmalloc_info() wrapper

The get_vmalloc_info() function was used to back the vmem_size()
function.  This was always problematic and resulted in brittle
code because the kernel never provided a clean interface for
modules.

However, it turns out that the only caller of this function in
ZFS uses it to determine the total virtual address space size.
This can be determined easily without get_vmalloc_info() so
vmem_size() has been updated to take this approach which allows
us to shed the get_vmalloc_info() dependency.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>