NFSD: Never return NFS4ERR_FILE_OPEN when removing a directory

RFC 8881 Section 18.25.4 paragraph 5 tells us that the server
should return NFS4ERR_FILE_OPEN only if the target object is an
opened file. This suggests that returning this status when removing
a directory will confuse NFS clients.

This is a version-specific issue; nfsd_proc_remove/rmdir() and
nfsd3_proc_remove/rmdir() already return nfserr_access as
appropriate.

Unfortunately there is no quick way for nfsd4_remove() to determine
whether the target object is a file or not, so the check is done in
in nfsd_unlink() for now.

Reported-by: Trond Myklebust <trondmy@hammerspace.com>
Fixes: 466e16f0 ("nfsd: check for EBUSY from vfs_rmdir/vfs_unink.")
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Cc: stable@vger.kernel.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>