fuse: fix fuse_file_lseek returning with lock held

This bug was found with smatch (http://repo.or.cz/w/smatch.git/).  If
we return directly the inode->i_mutex lock doesn't get released.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
CC: stable@kernel.org