fs: add missing unlock in default_llseek()

A recent change in linux-next, 982d8165 "fs: add SEEK_HOLE and
SEEK_DATA flags" added some direct returns on error, but it should
have been a goto out.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>