make_ext4fs: fix dentry padding when dentry size is 4088 or 4092

make_ext4fs: fix dentry padding when dentry size is 4088 or 4092

When the total dentry size is N * 4096 - 4 or N * 4096 - 8, there is
not enough room in the block to insert an extra padding dentry.
Instead, switch to always padding the previous dentry out to the end
of the block, which matches what the kernel does when creating new
directory entries.

Also fix dentry_size to return the size of all the dentries
without the padding, the len + 8 padding is incorrect, and the
DIV_ROUND_UP will round up to the correct number of blocks.

Change-Id: If11f90e4ee172c135a0eae2a50b8be213e2dd0dc