fs: make fiemap work from compat_ioctl

We don't need to do anything special with fiemap to make it work with a
64bit kernel from 32bit userspace, so just go down to do_vfs_ioctl().
Without this patch we were incorrectly failing to do fiemap with a 32bit
userspace on a 64bit kernel.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>