this adds a truncate option to clonefile, which is useful
for the test suite.

Signed-off-by: Kay Pedersen <[email protected]>
Original-patched-by: Rob Norris <[email protected]>

In some cases dbuf_assign_arcbuf() may be called on a block that
was recently cloned. If it happened in current TXG we must undo
the block cloning first, since the only one dirty record per TXG
can't and shouldn't mean both cloning and overwrite same time.
For example this can happen while writting to a file and cloning
a file at the same time.

This is also covered by a test. The filesize must be huge like
4G to trigger this bug. For that reason a 4G file is created
from /dev/urandom and than a file clone loop whith FICLONE
starts. After that the whole file is overwritten twice. The
test can trigger the bug most times but not every time.

Signed-off-by: oromenahar <[email protected]>