ext4: allow directory holes

The largedir feature was intended to allow ext4 directories to have
unmapped directory blocks (e.g., directory holes).  And so the
released e2fsprogs no longer enforces this for largedir file systems;
however, the corresponding change to the kernel-side code was not made.

This commit fixes this oversight.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org