Change return value for ValidateMountMode

1. rename it from ValidateMountMode to ValidMountMode
Because it's a function simply check mount mode is valid or not.
2. remove the rw check return value
It's not supposed to be combined into this function, and we already
have a function for that check.

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>