KVM: arm64: Stop handle_exit() from handling HVC twice when an SError occurs

Prior to commit defe21f4 ("KVM: arm64: Move PC rollback on SError to
HYP"), when an SError is synchronised due to another exception, KVM
handles the SError first. If the guest survives, the instruction that
triggered the original exception is re-exectued to handle the first
exception. HVC is treated as a special case as the instruction wouldn't
normally be re-exectued, as its not a trap.

Commit defe21f4 didn't preserve the behaviour of the 'return 1'
that skips the rest of handle_exit().

Since commit defe21f4, KVM will try to handle the SError and the
original exception at the same time. When the exception was an HVC,
fixup_guest_exit() has already rolled back ELR_EL2, meaning if the
guest has virtual SError masked, it will execute and handle the HVC
twice.

Restore the original behaviour.

Fixes: defe21f4 ("KVM: arm64: Move PC rollback on SError to HYP")
Cc: stable@vger.kernel.org
Signed-off-by: James Morse <james.morse@arm.com>
---
It may be possible to remove both this patch, and the HVC handling code
in fixup_guest_exit(). This means KVM would always handle the exception
and the SError. This may result in unnecessary work if the guest takes
the virtual SError when it is next restarted, but should be harmless if
SError are always re-injected using HCR_EL2.VSE.