Add some write-side stats to count buffered writes, buffered writethrough,
and writepages calls.

Whilst we're at it, clean up the naming on some of the existing stats
counters and organise the output into two sets.
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
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cc: linux-fsdevel@vger.kernel.org