x86/entry: Fix typo in comments for syscall_enter_from_user_mode()

Just to help myself and others with finding the correct function names,
fix a typo for "usermode" vs "user_mode".

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200919080936.259819-1-keescook@chromium.org