ksmbd: Fix unsigned expression compared with zero

The return value of the ksmbd_vfs_getcasexattr() is signed.
However, the return value is being assigned to an unsigned
variable and subsequently recasted, causing warnings. Use
a signed type.

Signed-off-by: Wang Ming <machel@vivo.com>
Acked-by: Tom Talpey <tom@talpey.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>