Proposing correction to posix_fadvise() usage in xlog.c

I believe I have a correction to the usage of posix_fadvise() in
xlog.c.  Basically posix_fadvise() is being called right before the
WAL segment file is closed, which effectively doesn't do anything as
opposed to when the file is opened.  This proposed correction calls
posix_fadvise() in three locations in order to make sure
POSIX_FADV_DONTNEED is set correctly since there are three cases for
opening a WAL segment file for writing.

I'm hesitant to post any data I have because I only have a little pc
with a SATA drive in it.  My hardware knowledge on SATA controllers
and drives is a little weak, but my testing with dbt-2 is showing the
performance dropping.  I am guessing that SATA drives have write cache
enabled by default so it seems to make sense that using
POSIX_FADV_DONTNEED will cause writes to be slower by writing through
the disk cache.  Again, assuming that is possible with SATA hardware.

If memory serves, one of the wins here is suppose to be that in a
scenario where we are not expecting to re-read writes to the WAL we
also do not want the writes to disk to flush out other data from the
operating system disk cache.  But I'm not sure how best to test
correctness.

Anyway, I hope I'm not way off but I'm sure someone will correct me. :)

Regards,
Mark