fs: btrfs: fix btrfs methods return values on failure

fs: btrfs: fix btrfs methods return values on failure

The btrfs implementation methods .ls(), .size() and .read() returns 1 on
failure, but the command handlers expect values <0 on failure.

For example if given a nonexistent path, the load command currently
returns success, and hush scripting does not work.

Fix this by setting return values of these methods to -1 instead of 1 on
failure.

Signed-off-by: Marek Behún <marek.behun@nic.cz>