seq: Attribute wakeups to seq_wait()'s caller, not to seq_wait() itself.

The poll_loop code has a feature that, when turned on manually or
automatically (due to high CPU use), logs the source file and line number
of the code that caused a thread to wake up from poll().  Until now, when
a function calls seq_wait(), the source file and line number logged was
the code inside seq_wait().  seq_wait() has many callers, so that
information is not as useful as it could be.  This commit changes the
source file and line number used to be that of seq_wait()'s caller.

I found this useful for debugging.

Signed-off-by: Ben Pfaff <blp@nicira.com>