Use __fpurge to ensure single-line writes to cache files

Use __fpurge to ensure single-line writes to cache files

On a recent Debian/Sid machine, I saw libc retrying stdio writes that
returned write errors.  The result is that if an export downcall returns
an error (which it can in normal operation, since it currently
(incorrectly) returns -ENOENT on any negative downcall), then subsequent
downcalls will write multiple lines (including the original line that
received the error).

The result is that the server fails to respond to any rpc call that
refers to an unexported mount point (such as a readdir of a directory
containing such a mountpoint), so client commands hang.

I don't know whether this libc behavior is correct or expected, but it
seems safest to add the __fpurge() (suggested by Neil) to ensure data is
thrown away.

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>
Signed-off-by: Neil Brown <neilb@suse.de>