btrfs: Sink extent_tree arguments in try_release_extent_mapping

This function already gets the page from which the two extent trees
are referenced. Simplify its signature by moving the code getting the
trees inside the function. No functional changes.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>