Don't assume that UFS on-disk format of a directory is the same as
defined by <sys/dirent.h>

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

Preallocate buffer for cookies. Cookies will be replaced with d_off
field in struct dirent at later point.

Skip entries with zero inode number.

Stop mangling dirent in ufs_extattr_iterate_directory().

Sponsored by:	Google Summer Of Code 2011