xfs: truncate transaction does not modify the inobt

[ Upstream commit a606ebdb859e78beb757dfefa08001df366e2ef5 ]

The truncate transaction does not ever modify the inode btree, but
includes an associated log reservation. Update
xfs_calc_itruncate_reservation() to remove the reservation
associated with inobt updates.

[Amir:	This commit was merged for kernel v4.16 and a twin commit was
	merged for xfsprogs v4.16. As a result, a small xfs filesystem
	formatted with features -m rmapbt=1,reflink=1 using mkfs.xfs
	version >= v4.16 cannot be mounted with kernel < v4.16.

	For example, xfstests generic/17{1,2,3} format a small fs and
	when trying to mount it, they fail with an assert on this very
	demonic line:

 XFS (vdc): Log size 3075 blocks too small, minimum size is 3717 blocks
 XFS (vdc): AAIEEE! Log failed size checks. Abort!
 XFS: Assertion failed: 0, file: src/linux/fs/xfs/xfs_log.c, line: 666

	The simple solution for stable kernels is to apply this patch,
	because mkfs.xfs v4.16 is already in the wild, so we have to
	assume that xfs filesystems with a "too small" log exist.
	Regardless, xfsprogs maintainers should also consider reverting
	the twin patch to stop creating those filesystems for the sake
	of users with unpatched kernels.]

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Cc: <stable@vger.kernel.org> # v4.9+
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Darrick J . Wong <darrick.wong@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>