Add a new "interruptible" flag showing that the caller is willing to be
interrupted by signals during the __gfn_to_pfn_memslot() request.  Wire it
up with a FOLL_INTERRUPTIBLE flag that we've just introduced.

This prepares KVM to be able to respond to SIGUSR1 (for QEMU that's the
SIGIPI) even during e.g. handling an userfaultfd page fault.

No functional change intended.
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20221011195809.557016-4-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>