xfs: replace on-stack xfs_trans_res with pointer in xfs_create()

There's no need to store a full struct xfs_trans_res on the stack in
xfs_create() and copy the fields. Use a pointer to the appropriate
structures embedded in the xfs_mount.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>