KVM: x86/mmu: Make HVA handler retpoline-friendly

When retpolines are enabled they have high overhead in the inner loop
inside kvm_handle_hva_range() that iterates over the provided memory area.

Let's mark this function and its TDP MMU equivalent __always_inline so
compiler will be able to change the call to the actual handler function
inside each of them into a direct one.

This significantly improves performance on the unmap test on the existing
kernel memslot code (tested on a Xeon 8167M machine):
30 slots in use:
Test       Before   After     Improvement
Unmap      0.0353s  0.0334s   5%
Unmap 2M   0.00104s 0.000407s 61%

509 slots in use:
Test       Before   After     Improvement
Unmap      0.0742s  0.0740s   None
Unmap 2M   0.00221s 0.00159s  28%

Looks like having an indirect call in these functions (and, so, a
retpoline) might have interfered with unrolling of the whole loop in the
CPU.

Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Message-Id: <732d3fe9eb68aa08402a638ab0309199fa89ae56.1612810129.git.maciej.szmigiero@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>