ext4: fix incorrect indentation of if statement

The indentation is incorrect and spaces need replacing with a tab
on the if statement.

Cleans up smatch warning:
fs/ext4/namei.c:3220 ext4_link() warn: inconsistent indenting

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>