ksmbd: fix unused err value in smb2_lock

CID 1502845 (#1 of 1): Unused value (UNUSED_VALUE)
value_overwrite: Overwriting previous write to err with value from
vfs_lock_file(filp, 0U, rlock, NULL).
6880                err = vfs_lock_file(filp, 0, rlock, NULL);
6881                if (err)
6882                        pr_err("rollback unlock fail : %d\n", err);

Reported-by: Coverity Scan <scan-admin@coverity.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>