powerpc: opal-core: pair alloc_pages_exact() with free_pages_exact()

powerpc: opal-core: pair alloc_pages_exact() with free_pages_exact()

opal-core allocates buffers with alloc_pages_exact(), but then
marks them as reserved and frees using free_reserved_area().

This is completely unnecessary and the pages allocated with
alloc_pages_exact() can be naturally freed with free_pages_exact().

Replace freeing of memory in opalcore_cleanup() with
free_pages_exact() and simplify allocation code so that it won't mark
allocated pages as reserved.

Link: https://patch.msgid.link/20260323074836.3653702-4-rppt@kernel.org
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>