Use fallocate() to create backing files for loopback devs
Use fallocate() to create backing files for loopback devs

nix 0.10 includes support for fallocate(), which is the slightly better
way to create a large, empty file.

fixes #319

Signed-off-by: Andy Grover <b7d524d2f5cc5aebadb6b92b08d3ab26911cde33@redhat.com>

stratis-storage/stratisd,mulkieran/stratisd,trgill/stratisd,stratis-storage/stratisd,stratis-storage/stratisd,trgill/stratisd,mulkieran/stratisd