x86/crash: Drop manual hooking of exception_table[]

x86/crash: Drop manual hooking of exception_table[]

NMI hooking in the crash path has undergone several revisions since its
introduction.  What we have now is not sufficiently different from the regular
nmi_callback() mechanism to warrant special casing.

Use set_nmi_callback() directly, and do away with patching a read-only data
structure via a read-write alias.  This also means that the
vmx_vmexit_handler() can and should call do_nmi() directly, rather than
indirecting through the exception table to pick up the crash path hook.

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