Use vmem_size() for 32-bit systems

On 32-bit Linux systems use vmem_size() to correctly size the ARC
and better determine when IO should be throttle due to low memory.

On 64-bit systems this change has no effect since the virtual
address space available far exceeds the physical memory available.

Reviewed-by: Tom Caputi <tcaputi@datto.com>
Reviewed-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #5347