iwlwifi: process multiple frames per RXB

The flow handler (hardware) can put multiple
frames into a single RX buffer. To handle
this, walk the RX buffer and check if there
are multiple valid packets in it.

To let the upper layer handle this correctly
introduce rxb_offset() which is needed when
we pass pages to mac80211 -- we need to know
the offset into the page there.

Also change the page handling scheme to use
refcounting. Anyone who needs a page will
"steal" it, which marks it as having been
used & refcounts it. The RX handler then has
to free its own reference and must not reuse
the page.

Finally, do not set the bit asking the FH to
give us each packet in a single buffer. This
really enables the feature.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>