ocfs2: Restrict inode changes in ocfs2_update_inode_atime()

ocfs2_update_inode_atime() calls ocfs2_mark_inode_dirty() to push changes
from the struct inode into the ocfs2 disk inode. The problem is,
ocfs2_mark_inode_dirty() might change other fields, depending on what
happened to the struct inode. Since we don't always have locking to
serialize changes to other fields (like i_size, etc), just fix things up to
only touch the atime field.

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