xfs: don't leak quotacheck dquots when cow recovery

If we fail a mount on account of cow recovery errors, it's possible that
a previous quotacheck left some dquots in memory.  The bailout clause of
xfs_mountfs forgets to purge these, and so we leak them.  Fix that.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>