x86/traps: Drop dummy_nmi_callback()

x86/traps: Drop dummy_nmi_callback()

The unconditional nmi_callback() call in do_nmi() calls dummy_nmi_callback()
in all cases other than for a few specific and rare tasks (alternative
patching, microcode loading, etc).

Indirect calls are expensive under retpoline, so rearrange the logic to use
NULL as the default, and skip the call entirely in the common case.

While rearranging the code, fold the exit paths.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>