xfs: factor out xfs_ioctl_setattr transaciton preamble

The setup of the transaction is done after a random smattering of
checks and before another bunch of ioperations specific
validity checks. Pull all the preamble out into a helper function
that returns a transaction or error.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>