IB/ipath: Use counters in ipath_poll and cleanup interrupts in ipath_close

ipath_poll() suffered from a couple subtle bugs.  Under the right
conditions we could leave recv interrupts enabled on an ipath user
context on close, thereby taking potentially unwanted interrupts on the
next open -- this is fixed by unconditionally turning off recv
interrupts on close.  Also, we now use counters rather than set/clear
bits which allows us to make sure we catch all interrupts at the cost of
changing the semantics slightly (it's now give me all events since the
last time I called poll() rather than give me all events since I called
_this_ poll routine).  We also added some memory barriers which may help
ensure we get all notifications in a timely manner.

Signed-off-by: Arthur Jones <arthur.jones@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>