NFSv4: Handle case where the lookup of a directory fails

NFSv4: Handle case where the lookup of a directory fails

BugLink: https://bugs.launchpad.net/bugs/1959879
commit ac795161c93699d600db16c1a8cc23a65a1eceaf upstream.

If the application sets the O_DIRECTORY flag, and tries to open a
regular file, nfs_atomic_open() will punt to doing a regular lookup.
If the server then returns a regular file, we will happily return a
file descriptor with uninitialised open state.

The fix is to return the expected ENOTDIR error in these cases.

Reported-by: Lyu Tao <tao.lyu@epfl.ch>
Fixes: 0dd2b474d0b6 ("nfs: implement i_op->atomic_open()")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>