libxfs: fix xfs_trans_alloc reservation abuse

libxfs: fix xfs_trans_alloc reservation abuse

Various xfsprogs tools have been abusing the transaction reservation
system by allocating the transaction with zero reservation.  This has
always worked in the past because userspace transactions do not require
reservations.  However, once we merge deferred ops into the transaction
structure, we will need to use a permanent reservation type to set up
any transaction that can roll.  tr_itruncate has all we need, so use
that as the reservation dummy.

[sandeen: tweaks to backport before libxfs updates]
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>