btrfs-progs: Change BUG() to use assert.

Change the definition of BUG() to use assert instead of abort to
provide information about the location of the issue.

Signed-off-by: Mitch Harder <mitch.harder@sabayonlinux.org>
Signed-off-by: David Sterba <dsterba@suse.cz>
Merge branch 'integration-next' into integration