generic: test file writers racing with FIDEDUPERANGE

generic: test file writers racing with FIDEDUPERANGE

Create a test to make sure that dedupe actually locks the file ranges
correctly before starting the content comparison and keeps them locked
until the operation completes.

[Eryu: added .gitignore entry, required "deduperace" binary in test]

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>