ocfs2: move ip_last_trans to struct ocfs2_caching_info

We have the read side of metadata caching isolated to struct
ocfs2_caching_info, now we need the write side.  This means the journal
functions.  The journal only does a couple of things with struct inode.

This change moves the ip_last_trans field onto struct
ocfs2_caching_info as ci_last_trans.  This field tells the journal
whether a pending journal flush is required.

Signed-off-by: Joel Becker <joel.becker@oracle.com>