Set errno for mkdirp() called with NULL path ptr

If mkdirp() is called with a NULL ptr for the path arg, it will return
-1 with errno unchanged.  This is unexpected since on error it should
return -1 and set errno to one of the error values listed for mkdir(2).

This commit sets errno = ENOENT for this NULL ptr case.  This is in
accordance with the errors specified by mkdir(2):

  ENOENT
  A component of the path prefix does not exist or is a null pathname.

Signed-off-by: Chris Dunlap <cdunlap@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #2248

lib/libspl/mkdirp.c

@@ -146,8 +146,10 @@ simplify(const char *str)
 	 *  bail out if there is nothing there.
 	 */
 
-	if (!str)
+	if (!str) {
+		errno = ENOENT;
 		return (NULL);
+	}
 
 	/*
 	 *  Get a copy of the argument.