fix use of posix_fadvise in xlog.c

Hi all,

I wanted to propose a fix for to xlog.c regarding the use of
posix_fadvise() for 9.1 (unless someone feels it's ok for 9.0).
Currently posix_fadvise() is used right before a log file is closed so
it's effectively not doing anything, when posix_fadvise is to be
called.  This patch moves the posix_fadvise() call into 3 other
locations within XLogFileInit() where a file handle is returned.  The
first case is where an existing open file handle is returned.  The
next case is when a file is to be zeroed out.  The third case is
returning a file handle, which may be the file that was just zeroed
out.

Does this look ok?

Regards,
Mark