acpi: Eliminate console msg if pstore.backend excludes ERST

This is patch 2/3 of a patch set that avoids what misleadingly appears
to be a error during boot:

ERST: Could not register with persistent store

This message is displayed if the system has a valid ACPI ERST table and the
pstore.backend kernel parameter has been used to disable use of ERST by
pstore. But this same message is used for errors that preclude registration.

In erst_init don't complain if the setting of kernel parameter pstore.backend
precludes use of ACPI ERST for pstore. Routine pstore_register will inform
about the facility that does register.

Also, don't leave a dangling pointer to deallocated mem for the pstore
buffer when registration fails.

Signed-off-by: Lenny Szubowicz <lszubowi@redhat.com>
Reported-by: Naotaka Hamaguchi <n.hamaguchi@jp.fujitsu.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>