If mtab support is disabled, disable ro/rw mtab checks

If mtab support is disabled, disable ro/rw mtab checks

Commit f9906424 introduced a check that should prevent different information in
mtab and /proc/mounts. The check can require significant amount of time, and for
systems without mtab support it has no sense.

Execute this code only on systems with mtab.

When a systems with large number of nodes (thousands) mount the filesystems
simultaneously, the time required for serialization causes the utimensat() to
take a large amount of time (tens of minutes) when a large number of nodes are
simultaneously updating the timestamp.

Signed-off-by: Stanislav Brabec <sbrabec@suse.cz>