quota: simplify two-level sysctl registration for fs_dqstats_table

There is no need to declare two tables to just create directories,
this can be easily be done with a prefix path with register_sysctl().

Simplify this registration.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <20230310231206.3952808-4-mcgrof@kernel.org>