ksmbd: missing check for NULL in convert_to_nt_pathname()

The kmalloc() does not have a NULL check.  This code can be re-written
slightly cleaner to just use the kstrdup().

Fixes: 265fd199 ("ksmbd: use LOOKUP_BENEATH to prevent the out of share access")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Acked-by: Hyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: Steve French <stfrench@microsoft.com>