Linux: fix permission op test for certain compilers
Some compilers complain that _inode is used uninitialised here.
Since this test requires -Werror, it causes the test to fail
and our permission op to be used in RCU mode, leading to lockups.
Initialise it to make the compilers happy.
Fixes a lockup seen on kernels 2.6.38+ on Gentoo and Debian.