xmalloc: remove struct xmem_pool init_region

xmalloc: remove struct xmem_pool init_region

This patch dispenses with the init_region. It's simply not necessary
(pools will still happily grow and shrink on demand in its absence) and the
code can be shortended by removing it. It also avoids the sole evaluation
of ADD_REGION without holding the pool lock (which is unsafe).

NOTE: The if statement that is removed from xmem_pool_destroy() has actually
      been bogus since commit 6009f4dd "Transcendent memory ("tmem") for
      Xen." when the allocation of the init_region was moved out of
      xmem_pool_create().

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Suggested-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>