Loopback (file backed by some other file on disk) is an important
use-case, and FUSE features such as passthrough and splicing aim to
reduce overhead.

Example: 

    Splice disabled:

    $ go test -bench FD -test.cpu=1,2
    goos: linux
    goarch: amd64
    pkg: github.com/hanwen/go-fuse/v2/benchmark
    cpu: Intel(R) Core(TM) i5-8350U CPU @ 1.70GHz
    BenchmarkGoFuseFDRead     	    1929	    597942 ns/op	3507.28 MB/s	    4290 B/op	     129 allocs/op
    BenchmarkGoFuseFDRead-2   	    2497	    421856 ns/op	4971.26 MB/s	   62500 B/op	     177 allocs/op
    PASS
    ok  	github.com/hanwen/go-fuse/v2/benchmark	3.510s

    Splice enabled:

    $ go test -bench FD -test.cpu=1,2
    goos: linux
    goarch: amd64
    pkg: github.com/hanwen/go-fuse/v2/benchmark
    cpu: Intel(R) Core(TM) i5-8350U CPU @ 1.70GHz
    BenchmarkGoFuseFDRead     	    1524	    750249 ns/op	2795.28 MB/s	    4132 B/op	      98 allocs/op
    BenchmarkGoFuseFDRead-2   	    2364	    463522 ns/op	4524.39 MB/s	   11719 B/op	     106 allocs/op
    PASS

in other words, the API complexity that splicing introduced was not
worth it: it actually slows down things.

Change-Id: I4f7519ab2cc5b0145d0f7f81f55ad20d544e9d87