Skip certain filesystem types on some platforms.

Skip certain filesystem types on some platforms.

On Linux the actual rootfs gets mounted over an initial
rootfs. The mountcache on Linux is populated with the
content of /proc/mounts and as such it will contain 2
entries for the rootfs. So when the fstype function
is used to lookup the type of filesystem based on
the dev_t value it will always find the rootfs first.
But people expect to find either ext2, ext3 etc.
so we added a table with per OS (currently only Linux)
the filesystem which should NOT be loaded into the
internal lookup table.