ext2fs: Don't assume that on-disk format of a directory is the same
as in <sys/dirent.h>

ext2_readdir() has always been very fs specific and different
with respect to its ufs_ counterpart. Recent changes from UFS
have made it possible to share more closely the implementation.

MFUFS r252438:
Always start parsing at DIRBLKSIZ aligned offset, skip first entries if
uio_offset is not DIRBLKSIZ aligned. Return EINVAL if buffer is too
small for single entry.

Preallocate buffer for cookies.

Skip entries with zero inode number.

Reviewed by: gleb, Zheng Liu
MFC after: 1 month