ksmbd: add low bound validation to FSCTL_QUERY_ALLOCATED_RANGES

Smatch static checker warning:
 fs/ksmbd/vfs.c:1040 ksmbd_vfs_fqar_lseek() warn: no lower bound on 'length'
 fs/ksmbd/vfs.c:1041 ksmbd_vfs_fqar_lseek() warn: no lower bound on 'start'

Fix unexpected result that could caused from negative start and length.

Fixes: f4415848 ("cifsd: add file operations")
Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Steve French <stfrench@microsoft.com>