libceph: use alloc_pg_mapping() in __decode_pg_upmap_items()

... otherwise we die in insert_pg_mapping(), which wants pg->node to be
empty, i.e. initialized with RB_CLEAR_NODE.

Fixes: 6f428df4 ("libceph: pg_upmap[_items] infrastructure")
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>