btrfs: use variable for io_tree when clearing range in btrfs_page_mkwrite()

We have the inode's io_tree already stored in a local variable, so use it
instead of grabbing it again in the call to btrfs_clear_extent_bit().

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>