btrfs: streamline btrfs_get_io_failure_record logic

Make the function directly return a pointer to a failure record and
adjust callers to handle it. Also refactor the logic inside so that
the case which allocates the failure record for the first time is not
handled in an 'if' arm, saving us a level of indentation. Finally make
the function static as it's not used outside of extent_io.c .

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>