quota: simplify the quotactl compat handling

Fold the misaligned u64 workarounds into the main quotactl flow instead
of implementing a separate compat syscall handler.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>