When rename("a", "b/.") is performed, target namei() call returns

dvp == vp. Rename syscall does not check for the case, and at least
ufs_rename() cannot deal with it. POSIX explicitely requires that both
rename(2) and rmdir(2) return EINVAL when any of the pathes end in "/.".

Detect the slashdot lookup for RENAME or REMOVE in lookup(), and return
EINVAL.

Reported by:	Jim Meyering <jim meyering net>
Tested by:	simon, pho
MFC after:	1 week

sys/kern/vfs_lookup.c

@@ -552,6 +552,12 @@ dirloop:
 	else
 		cnp->cn_flags &= ~ISLASTCN;
 
+	if ((cnp->cn_flags & ISLASTCN) != 0 &&
+	    cnp->cn_namelen == 1 && cnp->cn_nameptr[0] == '.' &&
+	    (cnp->cn_nameiop == DELETE || cnp->cn_nameiop == RENAME)) {
+		error = EINVAL;
+		goto bad;
+	}
 
 	/*
 	 * Check for degenerate name (e.g. / or "")