selinux: only filter copy-up xattrs following initialization

Extended attribute copy-up functionality added via 19472b69


("selinux: Implementation for inode_copy_up_xattr() hook") sees
"security.selinux" contexts dropped, instead relying on contexts
applied via the inode_copy_up() hook.

When copy-up takes place during early boot, prior to selinux
initialization / policy load, the context stripping can be unwanted
and unexpected.

With this change, filtering of "security.selinux" xattrs will only occur
after selinux initialization.

Signed-off-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: Paul Moore <paul@paul-moore.com>