Squashfs: fix f_pos check in get_dir_index_using_offset

One off error in the f_pos check.  If f_pos is 3 or less don't
bother reading the index because we're at the start of the
directory, and we obviously already know where that is on disk.

This eliminates an unnecessary read.

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>