Import BUILD_BUG_ON from Linux kernel source

Signed-off-by: Mark Fasheh <mfasheh@suse.de>
- Allow duperemove to write its hash tree out to a file. The hashes can be
  read back in at a later date so that the duplicate extent finding code can
  be run against it.

- This is primarily to ease testing of the duplicate extent finding code by
  allowing us to bypass the checksumming phase.

- We don't allow for dedupe from a hash file as duperemove has no internal
  context for doing the required file operations.

Signed-off-by: Mark Fasheh <mfasheh@suse.de>
Merge branch 'serialize'