xfs: assume the default quota limits are always set in xfs_qm_adjust_dqlimits

We always initialize the default quota limits to something nowadays, so
we don't need to check that the defaults are set to something before
using them.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>