ext4: add EXT4_INODE_HAS_XATTR_SPACE macro in xattr.h

When adding an xattr to an inode, we must ensure that the inode_size is
not less than EXT4_GOOD_OLD_INODE_SIZE + extra_isize + pad. Otherwise,
the end position may be greater than the start position, resulting in UAF.

Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Link: https://lore.kernel.org/r/20220616021358.2504451-2-libaokun1@huawei.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>