NFSv4: Handle case where the lookup of a directory fails

NFSv4: Handle case where the lookup of a directory fails

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 <[email protected]>
Fixes: 0dd2b474d0b6 ("nfs: implement i_op->atomic_open()")
Signed-off-by: Trond Myklebust <[email protected]>
Signed-off-by: Anna Schumaker <[email protected]>