ocfs2: Directory c/mtime update fixes

ocfs2 wasn't updating c/mtime on directories during dirent
creation/deletion. Fix ocfs2_unlink(), ocfs2_rename() and
__ocfs2_add_entry() by adding the proper code to update the struct inode and
push the change out to disk.

This helps rename/unlink on nfs exported file systems in particular as those
clients compare directory time values to avoid a full re-reading a directory
which hasn't changed.

ocfs2_rename() loses some superfluous error handling as a result of this
patch.

Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>