r23786: Use linux/dqblk_xfs.h rather than a private copy of this header in the

r23786: Use linux/dqblk_xfs.h rather than a private copy of this header in the
Samba3 tree.

This is neater, plus it avoids the need to get legal approval from SGI
to use their GPLv2-only code under GPLv3.

If/when SGI legal sort things out, we could consider adding back this
header for very old systems where linux/dqblk_xfs.h is not available.
(This used to be commit cb435543f84955be75368a3294bc6b627414d876)