generic: test splice() with pipes

generic: test splice() with pipes

Andreas Grünbacher reports that on the two filesystems that support
iomap directio, it's possible for splice() to return -EAGAIN (instead of
a short splice) if the pipe being written to has less space available in
its pipe buffers than the length supplied by the calling process.

This is a regression test to check for correct operation. Kernel
needs commit 3253d9d09337 ("splice: only read in as much information
as there is pipe buffer space") to pass.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>