number has caught up.  This means that
there is a theoretical possibility of livelock, if the request
generation number keeps getting bumped and the flush generation number
never catches up.  The fix is simple: use the request generation
number read at the beginning of the function.

Also, atomic_inc() followed by atomic_read() can be replaced with
atomic_int_return().  There's no real requirement for atomicity here
but we might as well shrink the code.

This bug was discovered using David Binderman's list of "set but never
used" warnings from icc.

Signed-off-by: Roland Dreier <rolandd@cisco.com>
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