ext4: allow ext4_truncate() to return an error

This allows us to properly propagate errors back up to
ext4_truncate()'s callers.  This also means we no longer have to
silently ignore some errors (e.g., when trying to add the inode to the
orphan inode list).

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>