system/physmem: take into account fd_offset for file fallocate

system/physmem: take into account fd_offset for file fallocate

Punching a hole in a file with fallocate needs to take into account the
fd_offset value for a correct file location.
But guest_memfd internal use doesn't currently consider fd_offset.

Fixes: 4b870dc4d0c0 ("hostmem-file: add offset option")
Signed-off-by: William Roche <william.roche@oracle.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20250122194053.3103617-2-william.roche@oracle.com
Signed-off-by: Peter Xu <peterx@redhat.com>