lindebugfs: Fix write

For write operation pseudofs creates an sbuf with the data.
Use this data instead of the uio as it's not usable anymore after
uiomove.

Reviewed by: hselasky
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D34114