KVM: arm64: Simplify return logic in user_mem_abort()

KVM: arm64: Simplify return logic in user_mem_abort()

With the refactoring done, the final return block of user_mem_abort()
can be tidied up a bit more.

Clean up the trailing edge by dropping the unnecessary assignment,
collapsing the return evaluation for kvm_s2_fault_compute_prot(), and
tail calling kvm_s2_fault_map() directly.

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