livepatch: account for patch offset when applying NOP patch

livepatch: account for patch offset when applying NOP patch

While not triggered by the trivial xen_nop in-tree patch on
staging/master, that patch exposes a problem on the stable trees, where
all functions have ENDBR inserted. When NOP-ing out a range, we need to
account for this. Handle this right in livepatch_insn_len().

This requires livepatch_insn_len() to be called _after_ ->patch_offset
was set.

Fixes: 6974c75180f1 ("xen/x86: Livepatch: support patching CET-enhanced functions")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>