KVM: guest_memfd: Add slab-allocated inode cache

KVM: guest_memfd: Add slab-allocated inode cache

Add a dedicated gmem_inode structure and a slab-allocated inode cache for
guest memory backing, similar to how shmem handles inodes.

This adds the necessary allocation/destruction functions and prepares
for upcoming guest_memfd NUMA policy support changes.  Using a dedicated
structure will also allow for additional cleanups, e.g. to track flags in
gmem_inode instead of i_private.

Signed-off-by: Shivank Garg <shivankg@amd.com>
Tested-by: Ashish Kalra <ashish.kalra@amd.com>
[sean: s/kvm_gmem_inode_info/gmem_inode, name init_once()]
Reviewed-by: Ackerley Tng <ackerleytng@google.com>
Tested-by: Ackerley Tng <ackerleytng@google.com>
Link: https://lore.kernel.org/r/20251016172853.52451-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>