Fix a memory leak in getcwd() that can occur after a filesystem
has been forcibly unmounted. If the filesystem root vnode is reached
and it has no associated mountpoint (vp->v_mount == NULL), getcwd
would return without freeing 'buf'. Add the missing free() call.

PR: kern/30306
Submitted by: Mike Potanin <potanin@mccme.ru>
MFC after: 1 week