btrfs: use the helper extent_buffer_uptodate() everywhere

btrfs: use the helper extent_buffer_uptodate() everywhere

Instead of open coding testing the uptodate bit on the extent buffer's
flags, use the existing helper extent_buffer_uptodate() (which is even
shorter to type). Also change the helper's return value from int to bool,
since we always use it in a boolean context.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>