KVM: guest_memfd: Add macro to iterate over gmem_files for a mapping/inode

KVM: guest_memfd: Add macro to iterate over gmem_files for a mapping/inode

Add a kvm_gmem_for_each_file() to make it more obvious that KVM is
iterating over guest_memfd _files_, not guest_memfd instances, as could
be assumed given the name "gmem_list".

No functional change intended.

Reviewed-by: Ackerley Tng <ackerleytng@google.com>
Tested-by: Ackerley Tng <ackerleytng@google.com>
Reviewed-by: Shivank Garg <shivankg@amd.com>
Tested-by: Shivank Garg <shivankg@amd.com>
Link: https://lore.kernel.org/r/20251016172853.52451-3-seanjc@google.com
[sean: drop .clang-format change]
Signed-off-by: Sean Christopherson <seanjc@google.com>