MFC r267351: Move root_mount_hold() functionality to separate mutex.
MFC r267351:
Move root_mount_hold() functionality to separate mutex.

It has nothing to share with mutex protecting list of mounted file systems.

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase