KVM: VMX: Reinstate __exit attribute for vmx_exit()

KVM: VMX: Reinstate __exit attribute for vmx_exit()

Tag vmx_exit() with __exit now that it's no longer used by vmx_init().

Commit a7b9020b06ec ("x86/l1tf: Handle EPT disabled state proper") dropped
the "__exit" attribute from vmx_exit() because vmx_init() was changed to
call vmx_exit().

However, commit e32b120071ea ("KVM: VMX: Do _all_ initialization before
exposing /dev/kvm to userspace") changed vmx_init() to call __vmx_exit()
instead of the "full" vmx_exit().  This made it possible to mark vmx_exit()
as "__exit" again, as it originally was, and enjoy the benefits that it
provides (the function can be discarded from memory in situations where it
cannot be called, like the module being built-in or module unloading being
disabled in the kernel).

Signed-off-by: Costas Argyris <costas.argyris@amd.com>
Link: https://lore.kernel.org/r/20250102154050.2403-1-costas.argyris@amd.com
[sean: massage changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>