ubifs: Fix 'hash' kernel-doc warning in auth.c

Fixes the following W=1 kernel build warning(s):

fs/ubifs/auth.c:66: warning: Excess function parameter 'hash' description in 'ubifs_prepare_auth_node'

Rename hash to inhash.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>