selinux: Clean up initialization of isec->sclass

Now that isec->initialized == LABEL_INITIALIZED implies that
isec->sclass is valid, skip such inodes immediately in
inode_doinit_with_dentry.

For the remaining inodes, initialize isec->sclass at the beginning of
inode_doinit_with_dentry to simplify the code.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>