cifsd: fix xfstests generic/504 test failure

If lock length in smb2 lock request from client is over
flock max length size, lock length is changed to flock max length
and don't return error response.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>