fix umask when noACL kernel meets extN tuned for ACLs

Fix insecure default behaviour reported by Tigran Aivazian: if an
ext2 or ext3 filesystem is tuned to mount with "acl", but mounted by
a kernel built without ACL support, then umask was ignored when creating
inodes - though root or user has umask 022, touch creates files as 0666,
and mkdir creates directories as 0777.

This appears to have worked right until 2.6.11, when a fix to the default
mode on symlinks (always 0777) assumed VFS applies umask: which it does,
unless the mount is marked for ACLs; but ext[23] set MS_POSIXACL in
s_flags according to s_mount_opt set according to def_mount_opts.

We could revert to the 2.6.10 ext[23]_init_acl (adding an S_ISLNK test);
but other filesystems only set MS_POSIXACL when ACLs are configured.  We
could fix this at another level; but it seems most robust to avoid setting
the s_mount_opt flag in the first place (at the expense of more ifdefs).

Likewise don't set the XATTR_USER flag when built without XATTR support.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>