selftests/mount: output WARN messages when mount test skipped

If /proc/self/uid_map doesn't exist, mount test case exits
wthout any warning. Fix it to print a warning that the test
is skipped because /proc/self/uid_map doesn't exist.

Signed-off-by: Zhang Zhen <zhenzhang.zhang@huawei.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>