Fix issue preventing using splice with reads (#505)

Fix issue preventing using splice with reads (#505)

Context: SPLICE_WRITE is not used with regular buffers
(i.e. when they are not file-descriptor backed buffers).
There is a bug which assumes file descriptors are used.
If the amount of data associated with those FD is lower
than twice the page size, SPLICE_WRITE is not utilized.
With regular buffers the aggregated size was always 0.
Because vmsplice (splice user pages to/from a pipe) is
called before splice in fuse_lowlevel.c, regular buffers
would also work with splice.

This patch prevents to fallback to non-splice enabled
copies when itheir is no FD involved.