[XFS] Don't wait for pending I/Os when purging blocks beyond eof.

On last close of a file we purge blocks beyond eof. The same code is used
when we truncate the file size down. In this case we need to wait for any
pending I/Os for dirty pages beyond the new eof. For the last close case
we are not changing the file size and therefore do not need to wait for
any I/Os to complete. This fixes a performance bottleneck where writes
into the page cache and cache flushes can become mutually exclusive.

SGI-PV: 964002
SGI-Modid: xfs-linux-melb:xfs-kern:30220a

Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Peter Leckie <pleckie@sgi.com>