Silence 'old_umask' uninit variable warning

Recent changes have caused older versions of gcc to mistakenly
flag 'old_umask' in vn_open() as an unitialized variable.  To
silence the warning initialize it.

  kernel.c: In function 'vn_open':
  kernel.c:525:6: error: 'old_umask' may be used uninitialized
  in this function

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

lib/libzpool/kernel.c

@@ -522,7 +522,7 @@ vn_open(char *path, int x1, int flags, int mode, vnode_t **vpp, int x2, int x3)
 {
 	int fd;
 	vnode_t *vp;
-	int old_umask;
+	int old_umask = 0;
 	char *realpath;
 	struct stat64 st;
 	int err;