selinux: parse contexts for mount options early

Commit b8b87fd9

 ("selinux: Fix selinux_sb_mnt_opts_compat()")
started to parse mount options into SIDs in selinux_add_opt() if policy
has already been loaded. Since it's extremely unlikely that anyone would
depend on the ability to set SELinux contexts on fs_context before
loading the policy and then mounting that context after simplify the
logic by always parsing the options early.

Note that the multi-step mounting is only possible with the new
fscontext mount API and wasn't possible before its introduction.

Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>