(..), calling readdir and looking for previous directory inode. In case of
.zfs/ directory this doesn't work, because .zfs/ is hidden by default, so it
won't be visible in readdir output.
Fix this by implementing VPTOCNP for snapshot directories, so __getcwd()
doesn't fail and getcwd() doesn't have to use readdir method.
This fixes /bin/pwd from within .zfs/snapshot/<name>/.
Suggested by: kib
Approved by: re (rwatson)
8e9fd65fbf