iwlwifi: pcie: move page tracking into get_page_hdr()

Move the tracking that records the page in the SKB for later
free (refcount decrement) into the get_page_hdr() function
for better code reuse.

While at it, also add an assertion that this doesn't overwrite
any existing page pointer in the skb.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>