[GFS2] Only set inode flags when required

We were setting the inode flags from GFS2's flags far too often, even when they
couldn't possibly have changed. This patch reduces the amount of flag
setting going on so that we do it only when the inode is read in or
when the flags have changed. The create case is covered by the "when
the inode is read in" case.

This also fixes a bug where we didn't set S_SYNC correctly.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>