tmem: Don't use map_domain_page for long-life-time pages

tmem: Don't use map_domain_page for long-life-time pages

When using tmem with Xen 4.3 (and debug build) we end up with:

(XEN) Xen BUG at domain_page.c:143
(XEN) ----[ Xen-4.3-unstable  x86_64  debug=y  Not tainted ]----
(XEN) CPU:    3
(XEN) RIP:    e008:[<ffff82c4c01606a7>] map_domain_page+0x61d/0x6e1
..
(XEN) Xen call trace:
(XEN)    [<ffff82c4c01606a7>] map_domain_page+0x61d/0x6e1
(XEN)    [<ffff82c4c01373de>] cli_get_page+0x15e/0x17b
(XEN)    [<ffff82c4c01377c4>] tmh_copy_from_client+0x150/0x284
(XEN)    [<ffff82c4c0135929>] do_tmem_put+0x323/0x5c4
(XEN)    [<ffff82c4c0136510>] do_tmem_op+0x5a0/0xbd0
(XEN)    [<ffff82c4c022391b>] syscall_enter+0xeb/0x145
(XEN)

A bit of debugging revealed that the map_domain_page and unmap_domain_page
are meant for short life-time mappings. And that those mappings are finite.
In the 2 VCPU guest we only have 32 entries and once we have exhausted those
we trigger the BUG_ON condition.

The two functions - tmh_persistent_pool_page_[get,put] are used by the xmem_pool
when xmem_pool_[alloc,free] are called. These xmem_pool_* function are wrapped
in macro and functions - the entry points are via: tmem_malloc
and tmem_page_alloc. In both cases the users are in the hypervisor and they
do not seem to suffer from using the hypervisor virtual addresses.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>