MFC r196309:

MFC r196309:

getcwd() (when __getcwd() fails) works by stating current directory, going up
(..), 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)

git-svn-id: svn://svn.freebsd.org/base/stable/8@196310 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f