udf: fix sparse warnings:

Fix sparse warnings:

  fs/udf/balloc.c:843:3: warning: returning void-valued expression
  fs/udf/balloc.c:847:3: warning: returning void-valued expression
  fs/udf/balloc.c:851:3: warning: returning void-valued expression
  fs/udf/balloc.c:855:3: warning: returning void-valued expression

Reported-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: Jan Kara <jack@suse.cz>