btrfs: tree-checker: fix bounds check in check_inode_extref()

The parentheses for the unlikely() annotation were put in the wrong
place so it means that the condition is basically never true and the
bounds checking is skipped.

Fixes: aab9458b ("btrfs: tree-checker: add inode extref checks")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>