fuse: use default writeback accounting

fuse: use default writeback accounting

commit 0c58a97f919c ("fuse: remove tmp folio for writebacks and internal
rb tree") removed temp folios for dirty page writeback. Consequently,
fuse can now use the default writeback accounting.

With switching fuse to use default writeback accounting, there are some
added benefits. This updates wb->writeback_inodes tracking as well now
and updates writeback throughput estimates after writeback completion.

This commit also removes inc_wb_stat() and dec_wb_stat(). These have no
callers anymore now that fuse does not call them.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Bernd Schubert <bschubert@ddn.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>