gfs2: Mark journal inodes as "don't cache"

Before this patch, journal inodes were considered regular inodes,
which meant that instead of evicting them, function iput_final would
just put them on the lru for later processing. If the file system
withdrew for whatever reason, the withdraw would never be seen until
the inode was evicted, which could be indefinitely.

This patch marks all journal inodes as "don't cache" which means
function iput_final will evict them immediately, allowing us to
properly recover the journal on other cluster nodes.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>