LinuxKPI: add kstrtoint_from_user() and DECLARE_FLEX_ARRAY()

Add an implementation of kstrtoint_from_user() based on the other
implementations and an attempt at DECLARE_FLEX_ARRAY() which works
for the driver needing it.

MFC after: 3 days
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D34231