tmem: Move global stats in a tmem_statistics structure

tmem: Move global stats in a tmem_statistics structure

And adjust the macro: atomic_inc_and_max to update the structure.

Sadly one entry: pool->pgp_count cannot use this macro anymore
so unroll the macro for this instance.

No functional change. The name has the 'tmem_stats' as it will
be eventually non-local.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Doug Goldstein <cardoe@cardoe.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>