support for splice(2) read

[ubnt-intrepid]

According to libfuse:

• When FUSE_SPLICE_READ flag is set (and write_buf is supplied), it attempts to splice at first and transfer a request message into the pipe.
• If the message length is shorter than sizeof(fuse_in_header) + sizeof(fuse_write_in) + pagesize, the entire of the request message is transferred into the userspace and hence the pipe is emptied.
• If a series of operations in data transfer with splice is failed, the receiving process is falling back to use read/readv.