Linux 3.11 compat: Replace num_physpages with totalram_pages

num_physpages was removed by
torvalds/linux@cfa11e08ed39eb28a9eff9a907b20913020c69b5, so lets replace
it with totalram_pages.

This is a bug fix as much as it is a compatibility fix because
num_physpages did not reflect the number of pages actually available to
the kernel:

http://lkml.indiana.edu/hypermail/linux/kernel/0908.2/01001.html

Also, there are known issues with memory calculations when ZFS is in a
Xen dom0. There is a chance that using totalram_pages could resolve
them. This conjecture is untested at the time of writing.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #273
Fix race in spl_kmem_cache_reap_now()

The current code contains a race condition that triggers when bit 2 in
spl.spl_kmem_cache_expire is set, spl_kmem_cache_reap_now() is invoked
and another thread is concurrently accessing its magazine.

spl_kmem_cache_reap_now() currently invokes spl_cache_flush() on each
magazine in the same thread when bit 2 in spl.spl_kmem_cache_expire is
set. This is unsafe because there is one magazine per CPU and the
magazines are lockless, so it is impossible to guarentee that another
CPU is not using its magazine when this function is called.

The solution is to only touch the local CPU's magazine and leave other
CPU's magazines to other CPUs.

Reported-by: DHE
Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #274