quota: Use register_sysctl_init() for registering fs_dqstats_table

register_sysctl_init() also prints information that may be useful for
further debugging when we fail to register sysctl table. Use it when
registering fs_dqstats_table.

Suggested-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>