userns: Pass a userns parameter into posix_acl_to_xattr and posix_acl_from_xattr

- Pass the user namespace the uid and gid values in the xattr are stored
   in into posix_acl_from_xattr.

 - Pass the user namespace kuid and kgid values should be converted into
   when storing uid and gid values in an xattr in posix_acl_to_xattr.

- Modify all callers of posix_acl_from_xattr and posix_acl_to_xattr to
  pass in &init_user_ns.

In the short term this change is not strictly needed but it makes the
code clearer.  In the longer term this change is necessary to be able to
mount filesystems outside of the initial user namespace that natively
store posix acls in the linux xattr format.

Cc: Theodore Tso <tytso@mit.edu>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andreas Dilger <adilger.kernel@dilger.ca>
Cc: Jan Kara <jack@suse.cz>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

fs/nfsd/vfs.c

@@ -480,7 +480,7 @@ set_nfsv4_acl_one(struct dentry *dentry, struct posix_acl *pacl, char *key)
 	if (buf == NULL)
 		goto out;
 
-	len = posix_acl_to_xattr(pacl, buf, buflen);
+	len = posix_acl_to_xattr(&init_user_ns, pacl, buf, buflen);
 	if (len < 0) {
 		error = len;
 		goto out;
@@ -549,7 +549,7 @@ _get_posix_acl(struct dentry *dentry, char *key)
 	if (buflen <= 0)
 		return ERR_PTR(buflen);
 
-	pacl = posix_acl_from_xattr(buf, buflen);
+	pacl = posix_acl_from_xattr(&init_user_ns, buf, buflen);
 	kfree(buf);
 	return pacl;
 }
@@ -2264,7 +2264,7 @@ nfsd_get_posix_acl(struct svc_fh *fhp, int type)
 	if (size < 0)
 		return ERR_PTR(size);
 
-	acl = posix_acl_from_xattr(value, size);
+	acl = posix_acl_from_xattr(&init_user_ns, value, size);
 	kfree(value);
 	return acl;
 }
@@ -2297,7 +2297,7 @@ nfsd_set_posix_acl(struct svc_fh *fhp, int type, struct posix_acl *acl)
 		value = kmalloc(size, GFP_KERNEL);
 		if (!value)
 			return -ENOMEM;
-		error = posix_acl_to_xattr(acl, value, size);
+		error = posix_acl_to_xattr(&init_user_ns, acl, value, size);
 		if (error < 0)
 			goto getout;
 		size = error;