d_off support

[Jack Halford]

Hello all,

	With the new 'd_off' field in struct dirent, we've (gandi.net) pushed a patch[1] to add support for it in multiple filesystems. Our specific usecase is for a userspace nfs server (nfs-ganesha) with zfs. At the moment we cache direntry offsets by calling lseek once per entry, with the patch we can get the offset directly from getdirentries calls which provides a significant speedup.

[1]: https://reviews.freebsd.org/D17917