Added special case when sorting hierarchically in MountInfoTable::read.
Added special case when sorting hierarchically in MountInfoTable::read.

It is legal to have entries in a `MountInfoTable` whose `entry.id` is
the same as `entry.parent`. This can happen (for example), if a system
boots from the network and then keeps the original `/` in RAM.
However, to avoid cycles when walking the mount hierarchy, we should
not treat these entries as children of their parent so we skip them.

This commit adds functionality to handle this case.

Review: https://reviews.apache.org/r/52596/

gsantovena/mesos,Aman-Jain-14/customizedMesos,Gilbert88/mesos,gsantovena/mesos,abudnik/mesos,yuquanshan/customizedMesos,andschwa/mesos,craimbert/mesos,asamerh4/mesos,yuquanshan/customizedMesos,asamerh4/mesos,reneploetz/mesos,gsantovena/mesos,zmalik/mesos,kaysoky/mesos,neilconway/mesos,zmalik/mesos,Gilbert88/mesos,zmalik/mesos,verizonlabs/mesos,gsantovena/mesos,neilconway/mesos,craimbert/mesos,abudnik/mesos,reneploetz/mesos,asamerh4/mesos,andschwa/mesos,kaysoky/mesos,gsantovena/mesos,jpeach/mesos,zmalik/mesos,dforsyth/mesos,Gilbert88/mesos,kaysoky/mesos,verizonlabs/mesos,neilconway/mesos,chhsia0/mesos,reneploetz/mesos,reneploetz/mesos,verizonlabs/mesos,yuquanshan/customizedMesos,zmalik/mesos,verizonlabs/mesos,dforsyth/mesos,andschwa/mesos,craimbert/mesos,andschwa/mesos,craimbert/mesos,jpeach/mesos,dforsyth/mesos,Gilbert88/mesos,dforsyth/mesos,chhsia0/mesos,verizonlabs/mesos,asamerh4/mesos,Gilbert88/mesos,kaysoky/mesos,Aman-Jain-14/customizedMesos,andschwa/mesos,craimbert/mesos,neilconway/mesos,gsantovena/mesos,asamerh4/mesos,asamerh4/mesos,abudnik/mesos,reneploetz/mesos,verizonlabs/mesos,reneploetz/mesos,reneploetz/mesos,yuquanshan/customizedMesos,Gilbert88/mesos,jpeach/mesos,abudnik/mesos,neilconway/mesos,andschwa/mesos,kaysoky/mesos,kaysoky/mesos,abudnik/mesos,Gilbert88/mesos,andschwa/mesos,dforsyth/mesos,dforsyth/mesos,zmalik/mesos,neilconway/mesos,craimbert/mesos,dforsyth/mesos,chhsia0/mesos,chhsia0/mesos,neilconway/mesos,gsantovena/mesos,asamerh4/mesos,abudnik/mesos,yuquanshan/customizedMesos,chhsia0/mesos,verizonlabs/mesos,chhsia0/mesos,Aman-Jain-14/customizedMesos,Aman-Jain-14/customizedMesos,yuquanshan/customizedMesos,jpeach/mesos,craimbert/mesos,Aman-Jain-14/customizedMesos,abudnik/mesos,kaysoky/mesos,Aman-Jain-14/customizedMesos,zmalik/mesos,jpeach/mesos,jpeach/mesos,yuquanshan/customizedMesos,jpeach/mesos,chhsia0/mesos,Aman-Jain-14/customizedMesos