Use file_dentry and file_inode wrappers

Fix bugs due to kernel change in torvalds/linux@4bacc9c9234c ("overlayfs:
Make f_path always point to the overlay and f_inode to the underlay").

This problem crashes system when use zfs as a layer of overlayfs.

Signed-off-by: Chen Haiquan <oc@yunify.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4914
Closes #4935