fuse: Fix seek offset calculation
fuse: Fix seek offset calculation

Since fh->pos is already incremented by n_bytes_skipped, check if
offset == fh->pos.

This error was hidden by the fact that the operation is retried when
-EIO is returned.  When it was retried, the stream was already in the
correct position and so no seek operation was required.

gicmo/gvfs,gicmo/gvfs,gicmo/gvfs,gicmo/gvfs