If an NFS operation expects a particular sort of object (file, dir, link,
etc) but gets a file handle for a different sort of object, it must
return an error.  The actual error varies among NFS versions in non-trivial
ways.

For v2 and v3 there are ISDIR and NOTDIR errors and, for NFSv4 only,
INVAL is suitable.

For v4.0 there is also NFS4ERR_SYMLINK which should be used if a SYMLINK
was found when not expected.  This take precedence over NOTDIR.

For v4.1+ there is also NFS4ERR_WRONG_TYPE which should be used in
preference to EINVAL when none of the specific error codes apply.

When nfsd_mode_check() finds a symlink where it expected a directory it
needs to return an error code that can be converted to NOTDIR for v2 or
v3 but will be SYMLINK for v4.  It must be different from the error
code returns when it finds a symlink but expects a regular file - that
must be converted to EINVAL or SYMLINK.

So we introduce an internal error code nfserr_symlink_not_dir which each
version converts as appropriate.

nfsd_check_obj_isreg() is similar to nfsd_mode_check() except that it is
only used by NFSv4 and only for OPEN.  NFSERR_INVAL is never a suitable
error if the object is the wrong time.  For v4.0 we use nfserr_symlink
for non-dirs even if not a symlink.  For v4.1 we have nfserr_wrong_type.
We handle this difference in-place in nfsd_check_obj_isreg() as there is
nothing to be gained by delaying the choice to nfsd4_map_status().

As a result of these changes, nfsd_mode_check() doesn't need an rqstp
arg any more.

Note that NFSv4 operations are actually performed in the xdr code(!!!)
so to the only place that we can map the status code successfully is in
nfsd4_encode_operation().
Signed-off-by: NeilBrown <neilb@suse.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>