fstrim: sync with busybox master (now an official applet)

fstrim: Needs to fire at the mp, not bd

It's a filesystem thing from this perspective.

fstrim: Indicate failure on error

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>

Change-Id: Icffb851cbe1895523e67e5ff18c67f9109a46a28