tmem: reorg the shared pool allocate path

tmem: reorg the shared pool allocate path

Reorg the code to make it more readable.
Check the return value of shared_pool_join() and drop a unneeded call to
it. Disable creating a shared & persistant pool in an advance place.

Note that one might be tempted to delay the creation of the pool even
further in the code. That however would break the behavior of the code
- that is if we ended up creating a shared pool and the
'uuid_lo == -1L && uuid_hi == -1L' logic stands  we still need to
create a pool - just not shared type.

Signed-off-by: Bob Liu <bob.liu@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>