KVM: x86: Don't disable IRQs when unregistering user-return notifier

KVM: x86: Don't disable IRQs when unregistering user-return notifier

Remove the code to disable IRQs when unregistering KVM's user-return
notifier now that KVM doesn't invoke kvm_on_user_return() when disabling
virtualization via IPI function call, i.e. now that there's no need to
guard against re-entrancy via IPI callback.

Note, disabling IRQs has largely been unnecessary since commit
a377ac1cd9d7b ("x86/entry: Move user return notifier out of loop") moved
fire_user_return_notifiers() into the section with IRQs disabled.  In doing
so, the commit somewhat inadvertently fixed the underlying issue that
was papered over by commit 1650b4ebc99d ("KVM: Disable irq while
unregistering user notifier").  I.e. in practice, the code and comment
has been stale since commit a377ac1cd9d7b.

Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
[sean: rewrite changelog after rebasing, drop lockdep assert]
Reviewed-by: Kai Huang <kai.huang@intel.com>
Link: https://patch.msgid.link/20251030191528.3380553-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>