geom_stripe: Cascade cantrim just like we do for gmirror

If any of the disks can support trim, cascade that up the
stack. Otherwise, trims won't pass through striped raid setups.

PR: 277673
Reviewed by: imp (minor style tweaks from bug report)

(cherry picked from commit ea2d874cca7cdfe6133c1835dadd8f0672723fa6)