vfs: deny copy_file_range() for non regular files

There is no in-tree file system that implements copy_file_range()
for non regular files.

Deny an attempt to copy_file_range() a directory with EISDIR
and any other non regualr file with EINVAL to conform with
behavior of vfs_{clone,dedup}_file_range().

This change is needed prior to converting sb_start_write()
to  file_start_write() in the vfs helper.

Cc: linux-api@vger.kernel.org
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>