userns: Convert struct dquot_warn

Convert w_dq_id to be a struct kquid and remove the now unncessary
w_dq_type.

This is a simple conversion and enough other places have already
been converted that this actually reduces the code complexity
by a little bit, when removing now unnecessary type conversions.

Cc: Jan Kara <jack@suse.cz>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>