KVM: arm64: Extract PFN resolution in user_mem_abort()

KVM: arm64: Extract PFN resolution in user_mem_abort()

Extract the section of code responsible for pinning the physical page
frame number (PFN) backing the faulting IPA into a new helper,
kvm_s2_fault_pin_pfn().

This helper encapsulates the critical section where the mmap_read_lock
is held, the VMA is looked up, the mmu invalidate sequence is sampled,
and the PFN is ultimately resolved via __kvm_faultin_pfn(). It also
handles the early exits for hardware poisoned pages and noslot PFNs.

By isolating this region, we can begin to organize the state variables
required for PFN resolution into the kvm_s2_fault struct, clearing out
a significant amount of local variable clutter from user_mem_abort().

Signed-off-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>