common/populate: fix S_IFDIR.FMT_BLOCK and ATTR.FMT_LOCAL for ext4 encryption

common/populate: fix S_IFDIR.FMT_BLOCK and ATTR.FMT_LOCAL for ext4 encryption

When ext4 encryption is enabled, the directory entries are encrypted
so we need to create fewer directory entries to guarantee that they
will all fit in a single block.

Also, the encryption metadata takes up extended attribute room so we
can only add a single xattr to guarantee that the xattrs will fit in
the inode.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>