Only store result of mount_all that mounted userdata

During boot sequence there can be multiple calls to mount_all. For the
userspace reboot to correctly remount userdata, we need to store the
return code of the one that was responsible in mounting userdata.

Test: adb root
Test: adb shell setprop init.userspace_reboot.is_supported 1
Test: adb reboot userspace
Test: checked dmsg
Bug: 166353152
Change-Id: Id0ae15f3bcf65fa54e4e72b76f64716c053af7fb